Lysine Doesn’t Help Cats with Viral Upper Respiratory Infections

A new systematic review has appeared discussing one of the most widely and longest used supplements in small animal medicine, the amino acid lysine, which is used to prevent and treat upper respiratory infections in cats caused by Feline Herpesvirus. I’ve only looked at the evidence concerning this supplement once in the past, and here was my conclusion at the time:

Lysine is an amino acid which is hypothesized to be useful in the prevention and treatment of Feline Herpesvirus (FHV-1) infections. This virus is extremely common, and many cats will be exposed and become infected as kittens. Clinical symptoms include sneezing, nasal congestion, and conjunctivitis, and they range from mild and self-limiting to very severe. Most cats will get over the initial infection, but many remain chronically infected. With suppression of immune function from stress, medication, or disease, the virus can re-emerge and cause symptoms again. A small subset of cats may develop chronic, ongoing symptoms associated with this infection. Vaccination reduces the severity of symptoms but does not prevent infection.

Lysine is proposed to interfere with the replication of FHV-1 by blocking the uptake of another amino acid, arginine. There are theoretical concerns that lysine supplementation could make cats arginine deficient, but experimental studies suggest this is unlikely in practice. So it appears to be safe, but does it work?

Well, maybe. For once, numerous studies have been done, but there is no clear, consistent pattern of results. Some show that oral supplementation is ineffective and might even make infection worse (Drazenovich, 2009; Rees, 2008; Maggs, 2007). Others do seem to demonstrate some benefit (Maggs, 2003; Stiles, 2002). So while lysine supplementation appears to be safe and there is a plausible rationale for its use, no definitive conclusion about its efficacy is justified.

The new review is less optimistic than my earlier assessment:

Sebastiaan Bol, Evelien M. Bunnik. Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review. BMC Veterinary Research 2015, 11:284

Taking all results discussed in this systematic review together, we conclude that lysine supplementation does not have an inhibitory effect on FHV-1 replication in the cat. The scientific data do not support lysine supplementation or additional research with cats, as has been advocated by some…Based on the complete lack of scientific evidence for the efficacy of lysine supplementation, we recommend an immediate stop of lysine supplementation for cats. Lysine supplementation is not effective to prevent cats from becoming infected with FHV-1, it does not decrease the chance of developing clinical signs related to active FHV-1 infection, and it does not have a positive effect on the clinical course of its disease manifestations. In fact, results from two clinical trials with cats even suggest that excess dietary lysine may have an enhancing effect on FHV-1 replication. Positive findings, either for HHV-1 or FHV-1, were the result of poor study design and could not be replicated in well-controlled, larger studies. Furthermore, the proposed mechanism of action of lysine-arginine antagonism does not work in cats and its result, lowering arginine levels, would be highly undesirable.

The table below from the article illustrates the evidence evaluated in this review:

lysine table 1

Interestingly, most systematic reviews are reluctant to make active recommendations even when the lack of evidence is quite clear. The general thinking is that one can only prove a therapy does work, one cannot prove it does not. Certainly, additional evidence can always appear that shifts the probabilistic conclusions of science in one direction or another. However, we must make practical decisions about the risks and benefits of treatments based on the evidence we have, and at a certain point we must be willing to decide that enough is enough, that adequate negative evidence has accumulated to merit rejecting a therapy. These authors clearly felt that point had been reached for lysine and FHV-1 infection.

The following table gives their reasoning for discontinuing the use of lysine. (Based on the lack of supporting evidence and my own negative clinical experience with it, I have not routinely recommended it for quite some time).


lysine table 2

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74 Responses to Lysine Doesn’t Help Cats with Viral Upper Respiratory Infections

  1. v.t. says:

    Honestly, I was hoping new research would lend a clue that it could at least help (that’s a special term, “help”, we’ve been told this for years). Now I understand why it doesn’t. I never once heard however, before this research, that “increased infection frequency and disease severity” could occur with it’s use – that’s scary.

    Unfortunately, clients won’t take well to this latest research. It has been available outside of the veterinary clinics for a very long time without a need for prescription, so just another win for the supplement manufacturers to keep making their profit.

    I’ve not met too many vets who aren’t compelled to recommend lysine – perhaps it’s one of those “it might not hurt” things when out of other options.

  2. dl says:

    I don’t want to argue with the research, but my experience is different. I adopted a 6 wk. old kitten with FVR four years ago. He was very ill until I started treating him with lysine, and the improvement was very rapid with lysine treatment. The disease went into remission for several months, then symptoms reappeared, and again I treated with lysine, and the symptoms vanished quickly. Since the age of a year and a half my cat has been entirely symptom free. If he ever becomes symptomatic again I will use the lysine once again. The research may not support it scientifically, but I for one do not argue with success, and it has proven to me that it works. I sincerely doubt that the remission of the disease was purely coincidental both times.

  3. skeptvet says:

    Of course you are inclined to believe your own experience above controlled research done by others. That is both a natural human tendency and the main reason for the persistence of ineffective treatments throughout history. Thousands of people have been just as certain of the effectiveness of bloodletting, homeopathy, Lourdes water, and every other treatment imaginable, yet those don’t work either. Viral rhinitis is a self-limiting disease. That means that with no treatment at all almost every cat will get better by themselves. Lysine probably won’t do any harm, but with as much evidence as there is that it doesn’t work, coincidence is a far more likely explanation for what happened to your cat than that the science is all wrong and it really works after all.

  4. dl says:

    …..and of course, you are inclined to believe what you believe despite evidence to the contrary, simply because you cannot replicate it in controlled tests. That doesn’t mean it doesn’t work. Homeopathy works as well, and I could cite you many instances in my own life and the lives of my animals wherein it has proven to be amazingly successful when all modern medicine had failed miserably in treating the condition over some period of time. This has happened far too many times for it to be due to a “self-limiting” condition or coincidence. If it is a placebo effect, that is fine with me – if it works, who cares? But since it works equally well or better on my animals as on me, I rather doubt that it is placebo, since the animals do not even know they are getting a remedy. Once again, I do not argue with success, and I don’t care how it works if it works and does no harm. People who have closed minds on such subjects will never learn anything about these things, however, because their insistence on being “Right” will not allow them to admit that there could be something they don’t understand and cannot prove scientifically that, nevertheless, works better than their allopathic medicines. I feel sorry for those who are more concerned with clinging to their current beliefs than with learning.

  5. skeptvet says:

    Actually, you are mistaken. While all of us are predisposed to trust our experiences even when more reliable evidence contradicts them, some of us recognize this bias and attempt to overcome it whereas others embrace the bias and reject the better evidence. Homeopathy is a wonderful example, but not in the sense you intend, because in fact it clearly does not work and cannot work, yet personal anecdotes are used to suggest it does despite the powerful evidence to the contrary.

    There can be no learning without openness to the possibility that our current beliefs are wrong. I am open to this, I simply require scientific evidence to justify rejecting my beliefs, not merely testimony about the beliefs or personal narratives of others. You appear unwilling to consider the possibility you might be wrong at all, and unconvinced that any scientific evidence could justify changing your mind. I suggest you are clinging a bit more tightly to your beliefs than I am, and it is your mind that is closed. It is ironic that you can accuse me of this when you clearly believe your observations are unchallengeable, but I see this type of irony a lot when discussing these things. You have committed firmly to unshakeable faith in your personal experiences, so nothing anyone else can say is likely to matter.

  6. v.t. says:

    dl, typically, an FVR cat can be a carrier cat for life (particularly if also having been infected with herpesvirus and/or calicivirus) – this means reinfection can reoccur (most typically during times of stress or immunocompromised status), and it also means the cat can infect other cats.

    For example, herpesvirus cats tend to have flare-ups during times of stress, and most often, those flare-ups are self-limiting, with or without treatment. If the symptoms do not improve within a certain time frame, of course, or are extreme, or additional symptoms present, additional treatment for those symptoms or secondary infections should be considered.

    I’m afraid that homeopathy does nothing for your cats or you. NOTHING. That’s not being close-minded, it’s being critical of claims that bear no evidence. Homeopathy has never, in over 200 years, been shown to work – the mere premise of it is simply ludicrous.

  7. dl says:

    Homeopathy does work, and it has been proven time and again. I was a skeptic towards it myself before it was proven to me how well it works. But I was a true skeptic, meaning that I was merely skeptical but had an open mind. You are not a true skeptic, because your mind is closed. Not only that but you are also rude and unpleasant when anyone disagrees with you. I feel very sorry for you, but I have no interest in continuing the conversation with you.

  8. skeptvet says:

    You really don’t seem to understand what “open-minded” means. It means fairly and thoughtfully considering the arguments and evidence before judging a claim. OI have done this for homeopathy (as you can see by the extensive literature search and analysis I have published here on the subject). The fact that I ultimately concluded the evidence proved it doesn’t work doesn’t make me closed minded, it just means I have made a decision after considering the arguments. What you mean by “open-minded” seems to be not ever deciding something is false, and that’s not what it means.

    As for rude, I don’t see any rudeness in my responses here. Taking offense when you can’t convince someone who disagrees with you is just a way of abandoning reasonable debate. There is nothing unpleasant about saying, “I hear you, but you are wrong.” You’ve certainly been far more rude to me by attacking my personality instead of my arguments and evidence.

  9. Nonya says:

    dl: Homeopathy DOES NOT work, which has been proven time and again. I could cite you many, many instances in my own life, the lives of my friends, and the many, many instances in all of our animals combined when homeopathy failed miserably and cure/relief only happened when seeking the help of modern medicine. Indeed, I can cite several instances where the time wasted mucking around with homeopathy was detrimental to humans and animals.

    My anecdotal evidence is just as valid as it yours, which is why I now rely on actual evidence rather than anecdotal reports and testimonies such as yours.

  10. Renee Perry says:

    I am a fan of science-based information, but not all research is done well. No matter the subject there is plenty of research that contradicts other research so it seems that some research is not done correctly. I question this study because my own experience with my cat shows dramatic improvements when he is given L-lysine. When he doesn’t get it his meow becomes very hoarse and then becomes normal quickly after he does finally get a dose. My behavior toward him is no different so this is not a placebo effect nor is it a misinterpretation on my part of what I am seeing or hearing. I have witnessed the effects of not getting L-Lysine and the return to normalcy after a dose too many times not to think that it not only works but works quite well.

  11. skeptvet says:

    There is no doubt that not all research results are reliable or reflect the truth. The issue isn’t whether or not research is perfect, it is whether it is more reliable that anecdotes. There are plenty of things that go wrong during a research study, but even more that can go wrong when you try something in a single animal and evaluate the results subjectively. Experiences like yours have convinced people that every therapy ever tried works, from bloodletting to prayer to homeopathy. Anecdotal evidence is a test nothing ever fails. And anecdotal evidence leaves us without direction when anecdotes conflict. I’ve given lysine to hundreds of cats, and many failed to improve or got worse. So whose anecdote reflects the truth?

    So while it is great that your cat feels better, it doesn’t prove anything, and it certainly isn’t more reliable evidence than the. existing research regardless of its flaws. Here are some articles that go into more detail about this.

    Why We’re Often Wrong
    Testimonials Lie
    The Role of Anecdotes in Science-Based Medicine
    Why We Need Science: “I saw it with my own eyes” Is Not Enough
    Don’t Believe your Eyes (or Your Brain)

  12. Icame across the Bol, Bunnick trial some time ago while researching for my own kitty. I might wonder if people who have seen an “improvement” with lysine are actually feeding a protein deficient diet (i.e. many commercial diets) which the lysine might help address.
    I realize its only one of the several “essential” amino acids for felines (I’m an equine nutritionist and horses have only three AA’s considered “essential”).
    My interest was piqued as I just got a feline “Clinical Update” email from DVM360 – which often has some info of interest – which featured an ad for L-lysine aimed at veterinarians.
    Just discovered your blog and think I will enjoy it immensely! I feel many aspects of alternative and complementary medicine are unnecessarily vilified but I feel this is in large part because of the role of unscrupoulous purveyors of snake oil – which, sadly, has included some physicians and veterinarians I have encountered.

  13. Sierra says:

    This article is false. I brought home a cat (very stupid mistake on my part) that was not being adopted and my other cat who has lived with me for 2 years and I have had since she was a baby, starting getting severely ill. She had discharge from her mouth, nose, and eyes. She was hiding in places and VERY close to death. I could not afford a vet at the time so I was frantically trying to find ways to help her. I came upon L LYSINE at a local Walmart. I bought cat protein milk and crushed up the lysine in the milk. I would crush up one or two pills in a small bowl and she would drink the milk. She did not want to drink it, I literally had to force her to drink it. After a couple of days of force feeding her these pills and milk, all her symptoms went away. Lysine truly works for feline respitory infections. I do not believe this article to be true. My cat would be dead right now without this amazing amino acid!

  14. skeptvet says:

    Unfortunately, life, and medicine, is too complicated for simple anecdotes like this to tell us what works and what doesn’t.

    Why Anecdotes Can’t Be Trusted

  15. Trish says:

    I agree with you 100 percent. My 7 yr.old Ragdoll has FHV. I give her one 250 mg. Lysine chew, broken up, in her wet good once a day. I have been doing this faithfully, everyday, for 4 and a half years. She has not had one flare-up or outbreak for that long. She is so healthy. The ones to buy are Vetri Science. You can get get them on Amazon for under $15.00 with free shipping for a pack of 120 which will last for about 4 months. Try it. One per day. Studies mean nothing to me. If it works, it works, and this does.

  16. Grace says:

    So evidence via medical journals mean nothing to you?

    Homeopathy can work and has saved the lives of many animals.

  17. skeptvet says:

    A single case report does not prove or disprove anything, especially when it doesn’t agree with literally hundreds of controlled studies of homeopathy over decades that have consistently and clearly failed to show evidence of any real benefit. The evidence has been reviewed and discussed extensively in previous posts.

  18. Cindy Larrabee says:

    Hello…I I’m not writing this comment to say anybody is right or wrong but I’m I’m saying that the cats I have have gotten very sick since my neighbor brought a bunch of cats home and threw them out the door she does this quite often..because they’re ill and she’s lazy but that’s besides the point.. I tried getting vets to help me but I’m on SSI, And cannot afford it..I did try asking if I could bring in 2 of them..and maybe I could get enuf meds for all of them..and make payments..(notan option because they want to diagnose each one separately.i just don’t have the money..I found the lysine on the site I get Thier pet food from…it takes a while but it does start helping them..the older cats are doing great..the younger ones take a little longer..I don’t give it everyday..the sickest ones have it 3 days then 1 or 2 days off .then 3 days …etc..and it does help as far as I can tell..but if I skip too many days they get worse. ..If they don’t get it at all they can so sick they won’t eat and so congested they they starve themselves cuz they can’t taste or smell they waste’s a horrible thing to see…I cried and prayed and then found this..there are so many unwanted animals..the abandoned and sick always find me..I do what I can ..and give them love to them..they all have places to sleep and blankets and food..I’ve tried to get help from all the agencies that say they will help..but sadly that seems to be a myth..or they are full up..and have to destroy them..I finally found a place to help with spay and neuter..but a couple have bad eyes so they won’t do Thier surgery..these two cats were born in insulation..I couldn’t get to them..but they found me with eyes swollen..puss filled..blind..they each have a good eye and a bad one..I have an ointment that helps a little but never cures that completely..and I know the virus they all have in thier systems dosent help..but they won’t spay I’m saving for wood and special meshbto build a big enclosed area where they can have toys grass and sunshine..and the neighbors cats can’t rape them..there are ferals that eat here too..and to be honest all of these are from the ferals..because the woman with catch and release is more concerned with have a big record for catch and release..than caring for the animals…the babies are left to die..neighbors find them and bring them here..where I hand feed them..and give them a’s not Thier fault they have no one or nowhere..and by the way that woman dumps the cats when she’s done getting the fixed..she dosent find homes or even bring them back to Thier colony..they disappear forever..animal control chgs. 50$ to pick up an animal..and then it’s destroyed..I wouldn’t do that if I had the money..that woman also takes ANY CAT that gets in the trap..spayed neutered or not…even if they are someone’s pets..I know this because she stole my 3 cats when I first moved here ..I ran her out of my yard 3 times(her and her cages with tuna in them) but she got all 3..later she got 3 I had hand raised from 2 weeks did make it back..after 2 months..infected and mal-nourished..yes I reported it to all the people I was supposed one cared..they passed me around like a VIRUS….none of the cats venture anywhere furter than my trailer and the two trailers from either side of one minds in the back of the park where we live..soon they will only be in my yard and in my house as I’ve found new ideas to keep them safe..and let them live out Thier lives in peace..I’m sorry I’ve gone on and on..I just get so sad that no one cares…I write not to say you are wrong but to say if Thier are any other poor folks who love and care for animals this lysine does help keep the virus from making them die a slow long death…and they are well enuf to play and eat and live..course maybe I’m totally wrong…maybe it’s just God listening to my tears and most likely may be the lysine..after all he blessed me with a son who gave me a phone and taught me to google things I need to learn about..he also blessed me with a research and help myself and those I love..

  19. v.t. says:

    Cindy Larrabee,

    Lysine isn’t going to help nor cure upper respiratory infections (you’re most likely dealing with feline rhinotracheitis with or without herpesvirus or calicivirus). The supplement *may* help with chronic herpesvirus, in that, it *may* lessen the severity of symptoms, but it cannot target secondary bacterial infections that are common with FVR. Antibiotics may be warranted here.

    Sounds like the TNR woman is part of the problem. You could contact your local/regional humane society chapter, shelters and rescue programs in your area for ideas on how to reach out to her for proper TNR handling (and print out educational brochures from AlleyCat Allies, ASPCA, etc and leave them on her doorstep)

  20. Houston Stackhouse says:

    I have produced over 300 show cats over the years. Of those cats countless kittens were raised that have developed symptoms and have died and some have lived. The cats I give lysine to dramatically outlive the cats that Dont get it. Running a controlled study , unless done over ten of years with hundreds of subjects are worthless.

  21. skeptvet says:

    Nonsense. Controlled studies are consistently more reliable that anecdotal experience, and while the sample size is important, there are many other methods to control for bias and error that make research superior to personal experience. A thousands anecdotes do not add up to data.

  22. Rosa says:

    I have experienced great results while using L-Lysine when cats have show symptoms of herpes virus. It always calms things down after just a few doses mixed in their good.

  23. Soothesay says:

    Hi Skeptvet, love your attitude and comments here. In the same spirit and paradigm I would like to suggest that thousands of anecdotes do add up to data, it is just unreliable data and difficult to parse. This isn’t a useful comment since you are practically right, just think it is fun to consider that given a large enough sample size of anecdotes, of people not purposefully being deceitful, one might be able to extrapolate useful data. A lot of this is being done in other areas under the moniker “big data” but it is a bit perilous as often the results given by using computer analysis of such data are scientifically useful, but the mechanism of their usefulness can’t be understood by people. That is to say, big data gives us things that work, but often we don’t know why they work. Scary stuff!

  24. skeptvet says:

    I guess we could play semantics about whether anecdotes, in whatever quantity, amount to data. As you point out, we agree that they have no effective control for bias, confounding, and other error, so ultimately they aren’t useful as data, only as a source of hypotheses to test, but I see what you’re saying.

  25. Peter says:

    Skepvet, your analysis is interesting and supported by ordinary common sense reasoning. You are of course attempting to challenge entrenched thinking about the value of homeopathic remedies, which is difficult for some to accept.

    However, you do pursue it too far. I’m happy that you acknowledge that at least some research falls short in that “not all research results are reliable or reflect the truth.” The issue for me is financing, and how “research” is undertaken initially. Today, we are confronted all the time with “research” used to endorse products and categories of commodities and most especially ingredients as a means to overcome public/consumer perceptions. The best example I can think of (that’s relevant here) is the use of inferior ingredients in pet foods. One can find innumerable “scientific” studies that will “prove” the value of corn (the most highly subsidized crop in the US) or other grains in pet foods, grains that are used as part of “least cost mix” protocols and used to dispose of “garbage” (I use as a dictionary definition, and according to patent information) in pet foods. Money and jobs are at stake and the industry needs to overcome ordinary common-sense science and biology. There are a lot of industry stakeholders here. There are “blogs” online where one can be confronted by “veterinarians” who will aggressively argue that corn is good in cat food, for example, and wherein the “veterinarian” will counter: “show me one study that proves that corn is bad…” etc. etc.

    Well, it’s fair to ask WHY there are innumerable studies that support the use of cheap fillers that are byproducts of industrial agriculture in pet foods. And its fair to ask WHY manufacturers and their compensated veterinary professionals sponsor or rely/comment on those studies. Your POV is absolutely valid, but its fair to counter that much “science” (and that word has specific meaning) can be flawed in that its purpose is self-limiting as pet food politics.

  26. skeptvet says:

    I would turn the question around and ask WHY, if there is research evidence to support the nutritional value of some of the ingredients you dislike, are you so confident they are terrible? What evidence do you have to support that claim that outweighs, in quality or quantity, the evidence that contradicts it?

  27. Rachel k says:

    I have also had amazing results and I’m the biggest sceptic around.. I take care of a stray whose feline herpes was so bad both his eyes were shut from conjunctivitis and the mucous around his nose was completely clogging him .. I bought lysine supplements and after years watching him suffer and feeling helpless finally within two weeks both his eyes were wide open .. and no nose discharge. I know the lysine did it because nothing else worked ..

  28. v.t. says:

    Rachel k, a case that severe requires antibiotics, which lysine isn’t, it’s a supplement!

    Please read up on feline viral infections, they are always susceptible to secondary bacterial infections, meaning, treat appropriately, not just with supplements!

    “Years” watching him suffer? Why didn’t you get him properly diagnosed early and on appropriate treatment?!

  29. David F says:

    So, after reading all the anecdotal comments about lysine helping, having two stray kittens recently adopted with the virus, and given lysine by vet to help them, i have come to the following conclusion:

    There must be something common amongst all of the people claiming lysine helped, as it “helped” with out cats. Then we ran out, and the cats haven’t had a flare up since we have had them. So what else did we do? We changed their diet. They eat boiled chicken/turkey. No store bought crap. And they are ravenous appetites. No sneezing or coughing. Doing great!

    One commenter above mentioned diet being low in protein and the lysine helping. I wouldn’t be surprised to find out all of the people with anecdotal stories of lysine helping feed their cat commercial grade death pellets. I believe that might be the connection to improved health and lysine.

    We have been giving our cats sprinkle of green vibrance and boiled chicken/turkey and they are doing better than ever.

  30. skeptvet says:

    I understand your logic, but unfortunately collections of anecdotes simply don’t add up to reliable data. For one thing, people with positive experiences have been shown to be far more likely to comment online than those with negative experiences, so the anecdotes you see don’t represent the experience most people are having. There are also many reasons why symptoms for a chronic waxing/waning disease like this change, and we only know about and notice some and ignore others, so our attribution of causation is biased by what we think is relevant to begin with and what we know and don’t know. Ultimately, the same kind of evidence, in the form of positive stories, has supported every medical therapy ever tried, including those which, like bloodletting, turned out to do no good and actually harm patients. Stories seem very straightforward and convincing, but sadly they aren’t reliable.

  31. skeptvet says:

    Yes, Ioannidis’ work has been really interesting and useful. The article you link to starts out with some pretty misleading, sensationalist BS about how Dr. Ioannidis has single-handedly shamed the entire domain of “science,” which is an unfortunate and silly characterization. Otherwise, though, it is a nice summary of the recent study and some of the issues with literature bias that lots of us in the world of EBM have been talking about for years. In 2012, for example, I pointed out a study showing that the systematic reviews of acupuncture research published in China are deeply flawed and almost always favor acupuncture. I talked about another such study in 2014. The first study on such regional bias came out in 1998, so again this is not a new subject invented by Dr. Ioannidis.

  32. Jake Matrix says:

    I work for a biotech that develops and manufactures POC tests for humans. Unfortunately, I have 4 cats sick right now with what started as what appeared to be uri but has turned more gastric now. As I was trolling the internet for ideas, I ran across this few year old debate. Read every line. I love the passion on all sides. Love it! I commend anyone willing to constructively and politely argue their side. It’s so healthy. In my line of work I have no choice but to be a “flip flopper ” when warranted. I don’t have the luxury of being closed minded as the consequences could be grave. I have found that the most talented, smartest and most inventive minds always consider alternatives. One study shows this then later another shows that. One very Vital trait to have is one I believe has been lost to the internet world….humility. It’s ok to say you were wrong and that you seen something or experienced something that changed your mind. This shows intelligence and creativity so important to advancements in everything. Culture tells us differently but it’s ok to change your mind based on something new you learn! Doesn’t make you weak….makes you stronger.

  33. skeptvet says:

    I agree completely. Of course, there is room for debate about the specific meaning of “open-minded.” Sadly, it is often used as a code for implying that someone who doesn’t agree with us on a specific point disagrees out of lack of fairness or willingness to consider our claims rather than because they have honestly considered, and ultimately rejected them. But science is all about humility. The humility to realize that our personal observations, however compelling they feel to us, aren’t a reliable guide to the true nature of the world. The humility to recognize that a group endeavor in which people with different perspectives, beliefs, and blind spots engage in a competition of ideas using the shared methods of science leads to truth much more reliably than any path we could follow alone. And the humilty, as you say, to change our minds when the evidence warrants it. Thanks for the comment.

  34. Kim says:

    I was surprised to learn that l-lysine is ineffective in treating upper respiratory and FHV-1 replication in cats. Two days ago, the vet recommended l-lysine supplementation for my 13 year old cat who was suffering from severe symptoms from a flare-up. The flare-up was likely precipitated by dental surgery, and the related stress. The night before starting the supplementation, she was too weak to get up and eat. It was a frighteningly fast descent. I had spoken with a replacement vet earlier in the day who assured me that it was probably just a flare-up and not to worry. After one dose, the following morning, she was doing much better. Today is morning 2 of supplementation and she is stronger than ever. Anecdotal evidence may be dangerous notably, when it contradicts sound scientific studies. That said, the recovery of my cat is very fast and I am hesitant to dismiss the role of the supplement in her improvement. Best regards.

  35. skeptvet says:

    This kind of anecdote is very common, and it often convinces us that whatever we are doing is working. However, there are so many reasons why one event following another doesn’t prove the first event caused the second that this concept even has a special name in the list of logical fallacies: Post hoc ergo proter hoc fallacy Fortunately, we have figured out that controlled scientific research is more reliable than this kind of trial and error.

    Here’s a bit of humor to illustrate the problem:

  36. BTParnum says:

    Hi skeptvet,
    Just want to say, this thread has been amazing! The overwhelming support of a substance proven to have absolutely no scientific evidence to warrant its use is lamentable yet completely expected.
    I like to imagine the “pro Lysine” posters as people who are 100% certain that climate change is a scientific fact, but when it comes to this… No way. Don’t believe it. It’s fake news.

    Anyway, please keep up the good work here!

    Now this:
    In the last couple of months our cat has these horrible sneezing fits and has mucous discharge from left nostril only. Eyes and ears look fine. Eats, drinks, and plays as normal. Indoor cat.
    Species: Feline (Domestic Short Hair)
    Sex: Female Spayed | Color: White And Brown
    Birth: 09/19/2016 | Age: 2y | Weight: 7 lb 10 oz

    Here is what has been done for her:

    1. Trip to the vet. Vet found temp was in range and, based on our testimony and suspicion of FHV shedding or URI, the cat was put on AmoxiClav (Clavamox) 62.5mg/mL 15mL for 7 days.
    (Note: The cat does have FHV but the vet is not considering this a “flare-up” or directly related.)

    2. While on Clavamox, did everything we could to help, including cleaning all surfaces and cat dishes, bedding, running steam baths to alleviate the nasal passages, etc…

    3. Saw no improvement after the 7 days, so we went back to the vet. Cat was then put back on Clavamox for 14 days as well as Dexamethasone SP (gen) 4mg/mL/mL and Famciclovir (gen) 250mg Tab.

    4. The 14 days ended last week and there was no improvement. As soon as we get some money we will return to the vet for the next steps, which likely include searching for nasal polyps, foreign bodies, and immunosuppressive disease.

    Before we go back to the vet, is there anything we should be considering that we’re not?

    Thanks in advance.

  37. skeptvet says:

    It sounds like you are doing a good job considering your options in concert with your vet. I will say that antibiotics are rarely helpful unless there is clear evidence of a secondary bacterial infection, so while I have no idea if they are appropriate for your cat, they are pretty widely overused on a “just in case” basis. Steroids are controversial because while they can sometimes reduce symptoms, they may also worsen or prolong viral infections by suppressing the immune response, so that is something to consider. Most cases of FHV run their course and stop, and antivirals may be helpful, but if the symptoms continue longer than expected, it is worthwhile to pursue testing for other issues, as it sounds like you are doing.

    Good luck!

  38. Kate says:

    What you have shown is not that Lysine Does Not Help Cats with Upper Respiratory Infections, but that Lysine May Help Cats with Upper Respiratory Infections, Other Times, it May Not.

    The more we know of systematic reviews, the more we discover how useless they are. Surely what this indicates is that “You need to take your cat to your vet and get advice and treatment.” As with all remedies, the first one may work, but your vet may need to try another, depending on the circumstances and your cat.

  39. skeptvet says:

    Well, what I have shown is that there is not consistent, replicable evidence that lysine helps, and there is good reason to believe, based on the core biology of FHVand lysine/arginine metabolism, that it probably doesn’t. The argument that negative clinical trial results should not be. used to justify abandoning an ineffective therapy because “it might work in some cases” is not a sound way to evaluate therapies because it ultimately precludes ever giving up on anything. And the idea that trial and error in individual patients is a goo guide to what works misses the whole point of why we need population-level trials in the first place, because such trial and error methods are notoriously unreliable.

  40. Tom C says:

    I appreciate skeptvet for responding to so many of the posts. I believe in statistically based medical research. It protects the population from scams. I am old enough to remember cancer patients making the trip to Mexico to take Laetrile and then coming back home to die. I will not be giving my cat lysine for her chronic nose discharge.

  41. Kate says:

    I’m not understanding your argument at all. It’s not just Lysine, but every drug has a piece of paper in the packet that says ‘it might not be suitable for’. Surely the clinical trials you speak of, are used to find this out. Doctors and vets then use their clinical expertise to make a decision whether to use it. Sometimes it works, sometimes it doesn’t.

  42. skeptvet says:

    It’s a question of proportioning our confidence in our treatments to the strength of the evidence. On one end of the spectrum is something like homeopathy, with an impossible theoretical rationale and decades of negative clinical trials showing no benefit. Sure, we can say that it “might” work for some type of problem or patient not yet tested in a clinical trial, but this is vanishingly unlikely.

    On the other end is something like antibiotics for urinary tract infection. We have an extensive knowledge of the basic biology, so our rationale for using a particular antibiotic guided by culture results for a particular infection is very strong. And we have abundant clinical trial evidence showing efficacy. Antibiotics don’t work perfectly in every situation, but the choice to use them is informed by strong evidence.

    Lysine is between these extremes. The basic rationale for using it seems to contradict what we know about the basic physiology of FHV-1 infection and lysine/arginine metabolism in cats, so on that level it is unlikely to work. Most of the clinical trials so far have failed to show any convincing benefit. Sure, we can’t say 100% that it will never work for anyone, but we can say that there is little reason to expect it to work. Since we have little else for FHV-1 infection (antivirals having limited efficacy and herpesvirus being so far incurable according to the best evidence we have), some people choose to try it. That’s fine, but the problem comes when we use such trial-and-error to make claims about whether it “works.” FHV-1 flares are usually self-limiting, and they are unpredictable in occurrence and duration. If we do nothing, most cats get better anyway. So if we give lysine, we can’t look at how often that cat gets flares or how long they last and say anything at all about whether or not the lysine did anything. That’s why we need clinical trials, and why we should take seriously the failure of lysine to show strong signs of benefit in clinical trials.

    Proponents of worthless or untested therapies often reject negative clinical trial results by saying “Well, it might still work for some patients.” Up to a point that’s true, but beyond a certain point it’s just a BS rationale for continuing to recommend a failed treatment. Lysine is in a middle, ground, like most treatments, where the evidence is not absolutely definitive, but what we have so far suggests it probably doesn’t work most off the time.

  43. Kate says:

    Your four paragraphs are quite confusing “something like homeopathy, with an impossible theoretical rationale and decades of negative clinical trials showing no benefit” is followed by “Antibiotics don’t work perfectly in every situation, but the choice to use them is informed by strong evidence.” then, “Most of the clinical trials [of Lysine] so far have failed to show any convincing benefit. Sure, we can’t say 100% that it will never work for anyone, but we can say that there is little reason to expect it to work.” concluding “Up to a point that’s true, but beyond a certain point it’s just a BS rationale for continuing to recommend a failed treatment. ”

    Homeopathy has decades of clinical trials showing its efficacy. Your rejection of it as ‘impossible theoretical rationale’ is illogical and well out of date. Some of the world’s foremost scientists have shown that the dilution argument fell long ago and succussion is an important aspect of that too.

    Antibiotics work, yes of course they do, but only as long as you don’t overuse them. They certainly do not work in ‘every situation’ as you claim.

    Because clinical trials show no ‘convincing benefit’ of Lysine, is possibly a reason to change the clinical trials, not the medication, if we know sometimes it works.

    Your subjective opinion, and your experience of what you now decide is a ‘failed treatment’ should not deter a more experienced medical practitioner from using it. In fact it would be immoral to suggest that they should.

  44. Sean Church says:

    I get it!
    What are the studies saying now?

  45. Dylan Prince says:

    I rather enjoy the fact that there is so much data to say that L-Lysine does not work and yet so many anecdotes say that it does, myself included, yet my cat’s case is slightly different. I sought modern veterinarian care immediately and the care worked for the most part save her eyes, and my cat was prescribed lysine as a “preventative measure”. Well her eye discharge cleared up after being given lysine but after these studies I’m more inclined to say that the medicine just took slightly longer to take effect and the self-limiting nature of virus ended her current bout of symptoms. However, I do find most online studies to be unreliable simply because many studies have been faked just to receive national attention. I’m not saying this article is given the sheer volume of studies done but I’m simply stating a fact. There are many cases of faked scientific and medical studies intended to just get national attention. For instance a study was published last year that said eating more chocolate would help you lose wieght. It was a study that was deliberately faked and gained national acclaim in order to probe that such studies do indeed exist and are quite common. But given the age of this study and no experts coming forth saying this article was false then I have no choice but to conclude it is indeed very accurate. Who knows, maybe in the near future we will know why L-Lysine appears to work for a select number of felines and not the majority of them. And perhaps give us definitive answers as to why so much positive anecdotal evidence exists. Is it just human misinterpretation of the cats immune system and lack of general knowledge of the virus? Or is there quite possibly something to the treatment we just don’t yet understand? Anything is indeed possible and to deny this is quite foolish.

  46. skeptvet says:

    The studies are reviewed in the post. Most don’t show much benefit, though a few suggest it might have some small effect.

  47. Deanne says:

    This has been an interesting thread. I find blind faith in science as bad as blind faith in anecdotes, since our understanding of the world is constantly being updated, changing, is subject to political influence and money etc.
    For those very reasons I’m astonished you cling so tightly to clinical trials which could EASILY be wrong. How are you to say the clinical studies have been done correctly and had enough data? The amount of positive anecdotes – which by the way ARE a form of ‘trials’ is enough for most doctors who have learned to be humble enough to say, “We don’t know if it’ll work for you, it might not but in some cases it has been seen or felt to help, and therefore you should try it.” Not everything is junk science!
    It’s a real mistake to ignore real people and real results, even if the spiffy research papers at the time don’t fully reflect the same. I realize this could apply to things like essential oils which I believe are total bunk – but when it comes down to the health and well being of a loved one or a pet, you have nothing to lose by trying and much to gain.

  48. skeptvet says:

    The very concept of “faith in science” shows you misunderstand the whole issue. Confidence in our conclusions should be proportional to the strength of the available evidence. Anecdotal evidence is very weak and has proven wrong consistently for centuries, so any claim based on it should only be held with a very low level of confidence. Clinical trials can vary from very strong to very weak evidence depending on many factors (design, sample size, outcome measures, statistical methods, and many, many others). The confidence we have in conclusions based on clinical trials can vary from low to very high.

    In the case of lysine, there are anecdotes both for and against it. Plenty of clients I have given to tell me it did nothing at all. So the anecdotal evidence is both weak and inconsistent. The clinical research evidence is also mixed, with the balance of the evidence leaning against. The basic rationale for how lysine should be helpful also turns out not to work, so that further weakens the evidence for it. On balance, I believe the evidence shows that lysine does little to nothing for most cats.

    There is no way to prove 100% that something never works for any patient. That’s not the standard. However the waste of time and resources on therapies that probably don’t help does ultimately harm patients. People will often use an ineffective treatment based on anecdotes and avoid other treatments with better evidence for unsound reasons (e.g. using glucosamine instead of NSAIDs for arthritis pain). Lysine is probably harmless (though there is some evidence it can worsen FHV-1 infections in some cases), and it is probably not effective. You are free to make whatever choice you like so long as you understand the evidence.

  49. Diane says:

    Deanne, I believe that what you think of as “science” isn’t quite accurate. Science by definition updates its understanding of a topic as new evidence becomes available. To compare that to “faith” is false equivalence. In the same way atheism is not faith in the absence of God– it’s the absence of faith in God. I.e. not a belief system or faith in itself.

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