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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59 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

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