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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12 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.

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