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. Alicia says:

    I just adopted a one year old cat from a high kill shelter. They did not tell me he was sick but I noticed immediately in the carrier as I was bringing him home that he was sneezing, and although purring, was very lethargic. Brought him immediately to the vet who prescribed Clavamox and l-lysine. The vet said the problem was probably viral but because he also had a slightly injured paw, the Clavimox couldn’t hurt. After one week, he is slowly starting to get better, eating tons, drinking, peeing, pooping in his litter box and finally, after 5 days, started grooming himself. He is still sneezing but less often and not really playing, he doesn’t seem to know how and I’m teaching him. I have no idea where he came from before the shelter, he’s over a year old, was not neutered or chipped but very trusting of humans and clearly an indoor cat.

    Since he came in my home, he has been loved like crazy, talked to constantly, brushed (which he loves!) and although he still sneezes seems on the road to recovery.

    I don’t know what specifically is helping him but my question is this: In the clinical trials, how were the cats treated? Were they in cages, lonely, with minimal human interaction? It seems the anecdotal success of lysine was in homes where the cats were loved, petted, and got a lot of personal attention. Could it be that the cats who were getting less love and attention in the lysine trials failed and the very fact that the cats had love and a reason to live actually help the lysine to work? Of course cats can live in the wild and have forever, but I truly believe that adding love and attention to these conscious beings can help their well being. All of the anecdotal evidence of lysine working on failing cats seems to come from loving pet owners. I personally believe that that factor can help. If the clinical trials were done in a clinical setting and the cats were psychologically depressed (which they can be!) that could have something to do with the lysine being less or non effective. I’m not a vet, but I believe the consciousness of a living being, of any species, can be a factor. Most animals crave love and attention and that can contribute to their wellness and perhaps even to the effectiveness of lysine.

  2. skeptvet says:

    Stress and handling can certainly have an impact on immune function. For what it’s worth, though, most medicines that truly work are effective regardless of that variable because the effect of environment is much, much smaller than the direct effects of the disease-causing organism and of medicines that work. For example, stress may increase the susceptibility to some bacteria infections, but antibiotics are usually effective regardless of the home environment of patients. The same is true for most pain medications, chemotherapy drugs, vaccines, and pretty much all proven medicines. While home environment does have an influence, it doesn’t make or break the effect of any medicine that is actually doing something.

    Similarly, I have seen plenty of well-loved housecoats over the years who have not responded to lysine. The anecdotes you hear are always biased in favor of the positive because people are more likely to share stories of success than failure, and positive anecdotes are repeated online by manufacturers selling products, while negative stories are not.

    So while I think it is a good question, I don’t think it is likely that this one variable invalidates all the negative research or validates anecdotal evidence for lysine.

  3. Doreen says:

    You wrote such a wonderful article on your fur baby. You sound like such a good mama to this kitty giving her all the love you can Breaks my heart how do you say animal suffer in life not knowing the touch of a gentle hand or love my vet just told me to try lysine because I have a person who has had three upper respiratory’s in the past year end it breaks my heart because Persians have a very narrow nasal passages and it’s hard for them to breeze to begin with I don’t know why they breed these beautiful animals of course for the almighty dollar to sell them I happen to inherit this cat from my daughter-in-law who got pregnant and didn’t want to cat yes she is a lot of work but when I saw her just covered with drawl soaking wet I panicked and rushed her to the vet he gave her a comvina Antibiotic injection and the next day she was fine we worry so much about our a little voiceless animals but I just want to commend you from what I read you are a wonderful person and very caring for your animal good luck with your little baby antibiotic injection and the next day she was fine we worry so much about our a little voiceless animals but I just want to commend you from what I read you are a wonderful person and very caring for your animal good luck with your little baby Doreen.

  4. Doreen says:

    God Bless you for caring for that poor sick feral cat. Wish there were many more people like you.

  5. K Polletta says:

    You are correct on research/trials. It takes large number of subjects for extended period of time. I can’t remember at the moment what course my instructor stated this fact but she was very insistent that we remember that fact and be sure to check subject numbers and length of study.

  6. angela page says:

    My cat was sneezing. gooey stuff coming out of nose and eyes. He looked dreadful. 500 mg lysine pill crushed daily added to water and injected in mouth cured him . End of story. And a happy ending too.

  7. skeptvet says:

    Glad your cat is better, but it has absolutely nothing to do with the lysine. Even those who think it is helpful don’t claim it works this way. Last time you had a cold and a stuffy nose, did you get better? Most of the time these things resolve oil their own. Yet another example of why anecdotes are misleading.

    Here is more about why anecdotes don’t tell us the real story.

    Why Anecdotes Can’t Be Trusted

  8. Sheila says:

    Lysine helped my cat, a foster cat and her newborn kittens. I saw it with my own eyes.

  9. skeptvet says:

    I’m glad your cat is well, but the problem is that “saw it with my own eyes” is an unreliable way of evaluating these things. Reality is complicated, things get better or worse for many reasons besides the one thing we are paying attention to, and people see what they hope and expect to see. Viral conjunctivitis, for example usually goes away on its own no matter what we do, so the fact that it went away after you started giving lysine doesn’t mean the lysine worked. If it was that simple, then we’d have to believe in Lourdes water and alien abduction and everything else anyone had ever see with their own eyes, and that’s never worked well for us in finding the truth about nature.

    Here is more about why anecdotes don’t tell us the real story.

    Why Anecdotes Can’t Be Trusted

  10. bc says:

    I don’t know who to believe. I have seen people say that these infections cleared up far faster once supplementing the l-lysine and that they weren’t making any progress whatsoever prior. You say that these things can clear up on their own, but what about stories of weeks and weeks with the running nose, sneezing, eyes issues and this supplement cleared it up right away? Perhaps you are correct. I really don’t know what to believe any more. Seems like everything eventually is bad or negative, even the stuff we are taught to be good.

  11. Debbie Milford says:

    so, Lysine doesn’t help with fhv, is the anything it does help with? Is there evidence it boosts the immune system, or are you thinking it’s just generally worthless? My vet has recommended trying it for my cat with chronic rhinitis

  12. skeptvet says:

    The evidence that it helps is poor, so our confidence in any benefit should be low. Risks are probably minimal (though there is some very weak evidence it can actually make things worse in some cases), so I don’t recommend it but I don’t argue strongly against it. The whole concept of “boosting the immune system” really doesn’t make any sense, and this isn’t how lysine would work even if it did. It specifically effects the ability of the virus to reproduce, rather than the immune response fighting the virus.

    There are antiviral drugs which can reduce symptoms in cats with severe FHV, but they are generally reserved for the worst cases since they have limited benefits and, like all effective treatments, some potential risks. Vaccination reduces clinical severity, though it doesn’t prevent or “cure” infection. Other than that and good overall health, there isn’t any specific treatment that has proven benefits.

    Here are the recommendations from a group of feline medicine specialists on FHV.

  13. Leslie says:

    Hi, Alice. I just saw your post while searching the Net. I believe that your theory is correct. I am NOT a vet, but wanted to tell you about my Sascha Belle. When I took her to the vet straight from the shelter, she was diagnosed as having one of the Feline Viruses. I can’t remember which, as it has been 14 years ago. She only had approximately 25% eyesight. She was precious. The poor thing had to go back and forth to the vet having blood drawn for weeks. One day, I took her and she was virus-free. (I don’t remember treating her with meds at all.) I told the vet that I gave her extra love and attention and she slept in my arms, next to my chest. I spoke to the vet about it. Me; “It has been proven that an animal’s love can help an ill person get better. Do you think that a person’s love can help an ill animal get better?” He said, “Maybe so.” Regardless of why, she was extremely loved and loved on. She passed away 2 months ago and took a piece of my heart with her. Good luck with your new baby. Hugs!

  14. Marie says:

    My cat is 12 and has FHV. She seemed really kinda the same for so long and I have not given her Lysine for several years. She is now noticeably stuffy in her nose and I fear she has trouble breathing. Will lysine help at all? It stresses her out way too much to bring her outside so I really don’t want to bring her to a vet of they can’t do anything for her. I’m struggling with what I should do.

  15. Christine Lank says:

    My cat is sneezing and I phoned the clinic who said start with lysine and call us back in couple days if no improvement. Doing my research first, I found this article and now not sure I want to give him some. Is there any thing you do recommend to your patients that is safe and that can help speed up healing? He is eating (though a bit less) and eye discharge is clear; just has a lot of sneezing and is not playing.

  16. skeptvet says:

    Like most viral respiratory infections, such as the common cold in humans, most cat URIs go away on their own, and there isn’t any proven effective treatment that makes them go away faster. Supportive care (aka TLC; keep warm, eat and drink plenty, rest, etc.) are helpful. Very ill cats may benefit from antiviral medications, and some with secondary bacterial infections may need antibiotics, but these come with potential risks as well as benefits, so we only use them when necessary.

  17. Christine Lank says:

    Ok thank you very much for taking the time to respond and for your advice.

    We’re only starting day 5 since sneezing began so I will give it some more time. His sneezing seems to be a little less so far but his appetite still low

  18. Kate says:

    Hello, I came across your website while searching for the reviews of l Lysine for cats. Recently, I adopted a kitten (11 weeks). She’s been sneezing 5-6times and started to show bit of runny nose. I also noticed that around her eyes are getting watery. Today, while she was having a nap, I saw spasms on her face several times and she looked pretty lethargic. I was so worried she might die during the night that I ran to the animal hospital and the doctor gave me Clavamox and l Lysine. In one of the Skeptvet’s writing, he/she mentioned that 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 FRV. Antibiotics may be warranted here. Does this mean that l Lysine can actually lesson the symptoms of herpesvirus but cannot cure the bacterial infection entirely unless cats were treated with antibiotics? I live in Canada but my mother language isn’t English so I just wanted to double check whether I understood correctly. I really appreciate all the comments from the writers. Although both Skeptvet and dl as well as other people had different point of view, I enjoyed reading their writings. Hopefully, my kitten feels better soon and my English get better! Thank you for all the information.

  19. skeptvet says:

    Thanks for your question.

    1. Some have suggested that lysine can help reduce the reproduction of FHV virus and reduce symptoms and outbreaks, but there is a lot of doubt about whether it works for this or not. In some studies, positive effects are seen and in others they are not.

    2. Not all cats with FHV infection get secondary bacterial infections, and most do not need antibiotics. If there are signs of a bacterial infection, lysine will nt have any effect on this, so an antibiotic would be needed.

  20. AJ says:

    I recall when this information first came out in 2015 and have been consistently puzzled as to why Lysine is still recommended by basically every vet despite this evidence. Is there something I am missing? Even brand new vets who have gotten the entirety of their education post this research finding are recommending lysine supplements for kitties with URIs and I just don’t understand! What are your thoughts on why this is still being recommended? Is it simply the “well, may as well try because it can’t hurt” approach?

  21. skeptvet says:

    That and the basic fact that if you give it and the cat gets better on its own, you are likely to give the supplement the credit, and no amount of data seems able to shake people out of anecdote-based beliefs.

  22. Pingback: Cat Herpes - Is Your Cat Suffering from this Sneaky Virus?

  23. Tony Nickonchuk says:

    I am so happy I found your site. I am a pharmacist who specializes in the evaluation of clinical evidence. I had always assumed that what I was being prescribed for my pets was strictly scientific. Unfortunately, that appears to not be the case. My cat recently had a kitty cold and she was getting quite sick. She stopped eating and drinking which is when I brought her in. I mostly just wanted her to get some SQ fluids. They gave her some, but they were only able to get in 12 mL because she got super stressed. They did give her a long acting injectable antibiotic, which I’m not the biggest fan of, but I was okay with it, as I know they can get secondary pneumonias from URTIs. What threw me off a bit was them sending me home with FortiFlora and lysine. The FortiFlora? Whatever. She likes the taste of it and seemed to down her whole packet of Hydracare when I mixed it with it, so, hey, hydration for the win. Gave me some Metacam too, which did seem to help with her symptoms. The lysine though? I was skeptical, and this just sealed the deal for me. The tube was $20, so whatever. It was more that they made it sound like a crucial component of the treatment, which it clearly isn’t. I haven’t used it, and it will be going in the garbage. And guess what? She has recovered. I think it was really the hydration that did it. She is a picky girl, and when she isn’t feeling well, she stops drinking and eating. As soon as she got the fluids she perked up, and then started taking in Hydracare, which probably helped further.

    For those saying their cats got better after lysine, I will repeat something my infectious diseases professor used to say when she taught us about using antibiotics for human colds. Patients will recover in 14 days without antibiotics, or 2 weeks with them.

  24. AJ Dahl says:

    I was giving our elderly cat expensive Lysine paste for over a year, but she continued to have herpes cold like flareups and generally deteriorating health. As her health deteriorated, I researched lysine and found a peer reviewed study questioning the benefits my vet had claimed. I stopped the lysine, and 2 months later her health has greatly improved. I believe part of the downturn in her health had been from the side effects of lysine.

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