Upper respiratory infections are common in cats, and the symptoms of these kitty colds – sneezing, runny nose, runny eyes, lack of appetite, fever – are very much the same as what we experience during a cold. In addition to conventional treatments such as antibiotics or antiviral drugs, L-Lysine supplementation is often recommended to treat symptoms and prevent a recurrence. According to a recent study, more than 90 % of veterinarians in cat hospitals recommend L-Lysine supplementation.

What causes upper respiratory infections?

Upper respiratory infections are caused by either a viral or bacterial infection. The two most common viruses that cause upper respiratory infections in cats are herpes and calici. Bacterial infections are most commonly caused by mycoplasma pneumoniae, bordatella bronchiseptica, and chlamydia psittaci bacteria.

Study finds L-Lysine supplementation is ineffective and potentially harmful

A recent study looked at infections caused by herpes virus 1, and found that L-Lysine supplementation was not effective in preventing or treating herpes infections in cats. L-Lysine supplementation also didn’t decrease the development of clinical signs, nor did it aid in treating symptoms.

L-Lysine does not have antiviral properties, but researchers found that it does lower arginine levels. Arginine is an essential amino acid that is involved in a number of different functions, including wound healing, helping the kidneys filter waste products, and maintaining immune and hormone function. Cats cannot synthesize this amino acid themselves, and arginine deficiency can result in hyperammonemia (a metabolic disturbance characterized by an excess of ammonia in the blood,) which can be fatal.

Finally, and most importantly, several clinical studies with cats have shown that L-lysine is not only not effective for the prevention or the treatment of feline herpesvirus 1 infection, some even reported increased infection frequency and disease severity in cats receiving L-Lysine supplementation.

Should you stop L-Lysine supplementation?

The study’s conclusion recommends an immediate stop of L-Lysine supplementation because of the complete lack of any scientific evidence for its efficacy. Fern Crist, DVM, a former board member of the American Association of Feline Practitioners, says she can’t count the number of cats she’s put on L-Lysine over the more than 30 years she’s been practicing, but says that, based on these findings, “I’m inclined to stop using L-Lysine, at least for the time being, and wait for more information.” Dr. Andrea Tasi, a feline veterinarian and owner of Just Cats Naturally, doesn’t recommend L-Lysine anymore. “Over time I came to see it didn’t do much for most cats,” she says.

You can read the full text of the study here.

66 Comments on Study Finds L-Lysine Does Not Decrease Symptoms of Feline Upper Respiratory Infections

  1. Just to add more to this discussion, I’ve recently had experience with one of our cats developing the sniffles. We had recently adopted a very friendly kitten from a local shelter, three days after getting the kitten home, it developed the sneezing, runny eyes symptoms. Our usual vet was booked solid for the next two days, so we called around and got an appointment at a newly established clinic. New vet, just setting up his practice and I was surprised that he had separate treatment rooms for Cats and Dogs.
    Now to the visit with the new vet:
    A. it’s most likely a virus – knew that
    B. antibiotics won’t work – that too
    C. give him plenty of food, water – ditto
    D. prescribed an appetite stimulant – good idea
    E. Vet suggested adding Lysine to his diet, and he’s seen it work before.

    So I go out and get some lysine chews at the local pet food chain.

    Two days later, the kitten sneezes blood and my daughter calls the Vet, now the original vet is only open Monday to Friday and it’s Saturday.
    Our usual vet is open and takes the kitten right away, same diagnosis except, he gives the kitten liquid antibiotics and a bottle for us to take home. It seems he’s seen this condition a lot this year and the cats he’s seen all develop secondary infections and he wanted to head that off. (Funny, I remember hearing the same thing from my doctor about 10 years ago.)
    When my daughter mention the lysine, he said he was just about to mention that same instruction. He explanation was most of the commercial cat foods were deficient in lysine and adding it into the diet corrected the problem. He also listed people foods that are high in lysine and we could cover the deficiency that way, suggesting unflavored or plain yogurt if the cat liked it.
    Add to the above instructions, a trip into a steamy shower. He called it, the shower nebulizer. Not knowing how the kitten would react, we did this with some caution, he LOVED it. He sat on the shower seat and didn’t want to leave until the steam had completely dissipated.

    4 days down the road and the kitten is much improved, one of the other cats started down the same path. We started the same treatment and headed off the worst of the symptoms on the other cat.

    Normally I’m skeptical of anecdotal information, but this worked for us in two cases, if lysine deficiency exacerbates the virus symptoms, then this seems a reasonable treatment. If you don’t want to use a packaged supplement, look up foods high in lysine and if your cat like one or two, add that to his diet.

    As too the research, they may be chasing after the wrong question.
    “does lysine impact the viruses.”
    “is susceptibility to viruses or poor immune response a symptom of lysine deficiency in cats.”

  2. Who performed the study? Usually when study is mentioned to criticize a supplement the name of the researchers, universities or private funded research. In my opinion there is no research today advising not to give your cat lysine. Don’t believe everything you read on the internet. Look up studies by reputable learning institutions such as A & M colleges.

  3. In my opinion, you should quit giving your cats L-lysine. If you read enough about it you will find that a very large percentage of cats taking it for one condition die from another, kidney failure. This happened to us, btw.

    • Does anyone know about the effectiveness of collidal silver for upper respiratory and mouth sores in cats?

      I was about to purchase L-lysine for these conditions untill I read this article.
      I personally use silver for immune health and am hoping the cats can benefit as well.

    • Really? Where did you find that “a very large percentage of cats taking it for one condition die from another, kidney failure?” I have never heard that fact. Also, I have successfully used l-Lysine to treat eye conditions in three cats. There were never any side effects. I recommend people try it.

  4. I think more real research needs to be done on this subject before we condemn the use of L-Lysine for the treatment of FHV in cats. All that was done here was a compilation and research of previous studies. I also want to point out that some of the studies conducted previously may have been funded by companies who produce L-Lysine supplements. If you are going to insist that we stop giving our cats L-Lysine that most people believe to be working than give us something else to use. Or better yet, spend some of that time, energy, and money that you put into proving it has no effect into finding a cure for FHV.

  5. My cat had cold symptoms for about a year (after two separate rounds of antibiotics), then I tried the L-lysine and he is much better! Prior to treatment his eyes were all runny and that has cleared up completely (within a few days). I would say that after a year of symptoms and then taking L-lysine, it’s obvious that it worked!

    • Spanky — our Siamese mix — had identical symptoms for nearly three years with several rounds of antibiotics given over that time. Results were mixed with trace symptoms continuing. Finally went for the L-Lysine and voila! The symptoms disappeared after just three days. We haven’t given him any additional LL since so I’d imagine the risk of side effects is minimal to nil.

      • Thank you for providing the length of time your cat was given the LLysine. This is very important and the studies listed in the article were not specific about that. I know L-Lysine is recommended for chronic conditions and maybe using it long term is what creates the kidney problems. Although kidney problems can develop from bad food. So we never get enough information for taking care of our fur-balls unfortunately. But when details are provided like are provided here, we can make better decisions. Thank you!

  6. Not sure how many cats they used in this trial? While living in the Middle East I treated many of the stray cat population with l-lysine, I can definitely say it made a huge difference, some were asymptomatic after a couple of weeks treatment.

  7. I also question the L-arginine finding in the study mentioned. Both L-lysine and L-arginine are essential amino acids found in food, especially meat, poultry, etc. There is no indication as to what diet the cats in the study were on. Cats are carnivores and should be eating a whole animal diet. If they are getting enough L-arginine and the other essential amino acids and elements in their diet, there shouldn’t be a problem with taking extra L-lysine. However, if they’re on a low protein diet, or dry food, that’s another story. Anyone who takes one isolated supplement without the rest of the complex that comes with it in nature may be facing a deficiency. Humans can have genetic deficiencies of certain essential elements or genetic illnesses that cause deficiencies that can be passed down in families and if not supplemented can cause health problems. I’m sure the same is true for other species, like cats.

  8. I started my 12-year-old cat on L-Lysine for stomatitis a couple of years ago. Her stomatitis cleared up within a few months. L-Lysine works for stomatitis, especially used with seaweed for cats you can buy that helps with tartar and plaque on a cat’s teeth. My cat doesn’t have other health problems, like herpes, infections, etc., but I don’t doubt L-lysine works for those. I will not stop giving it to her. If you Google stomatitis in cats you will find a website by someone about how L-Lysine together with seaweed eliminated her cat’s (Lilly) severe stomatitis. It’s very detailed and there are before and after pictures. It’s very interesting. Anyone can say there’s a study that confirms their particular viewpoint but there are plenty of studies out there that are biased or don’t meet scientific criteria. I like to do my own research. I prefer to trust real evidence. Pharmaceutical drugs are very harmful to cats and can kill them and I like to look for alternatives before resorting to that. Pharmaceuticals have their place in medicine but medicine isn’t a science and doctors and vets are not scientists.

    • I know the site that you are talking about and her cat with stomatitis recently passed of severe kidney failure. Something like that makes me wonder if there is something to this study.

      I just started my cat with stomatitis on l-lysine powder so I really hope this is not true. Going to look into l-arginine supplementation for cats and how much to give to be safe.

  9. I think you really need to take into account how your cat reacts. Just like people, some meds can work well for you but not someone else. It doesn’t hurt to give l-lysine a try and see if you get results. As you can see from the tons of comments above, some cat owners swear by this supplement and continue to use lysine to help with their cat’s symptoms. As with any medications, you should speak with your cat’s vet and determine the best options for your pet.

    • I’m back again…was giving all my cats L-Lysine as they all had congestion, eye infections etc. After a lot of Human Trauma here I had stopped as all had cleared up. My 17 year old cat though within a week reverted back and I almost lost her…she looked like death. I started the L-Lysine again and a week later her discharge is running clear again. She is demanding food…lol and even if I miss a dose just by a few hours she starts looking not so good…I don’t need a study of questionable origins to believe this works!

  10. I beg to differ. I have my cats on lysine to control herpes. It is the “ONLY” thing that works. If you can get your cat to eat any of the different types of lysine treats on the market, it’s gold. Try it and see. No vet bills, antibiotics or suffering for your cat. Increase dose for a few days for flare ups and a daily maintenance of the usual dose. If you have a really big cat, you may have to adjust the daily dosage to compensate for poundage. Trust me. If it’s an arginine problem, than just supplement I guess, that’s the first I’ve heard of an arginine absorbtion problem with lysine. People take lysine too, it’s just we don’t need arginine like cats. Just like people take Prilosec and others called PPI’s for heartburn though studies show if taken for many years, it may be linked to osteoporosis due to absorbtion problems. With all that said, you won’t need a vet or medications or anything like that, it works, hands down. I’ve taken my cats to the vet for herpes, they prescribe antibiotics and that’s all they have. That does not work for herpes. You will run up vet bills, pay for meds that ultimately don’t work and during all this back and forth, you pet is suffering. Anyway, try it, it works for the facial, ear and lung issues that occur with the herpes. It can be real suffering for your cat if they have it bad enough and lysine is the only thing that will work for you, period.

  11. My cat developed a serious eye infection in one of his eyes and was prescribed a lengthy course of antibiotics. The infection would reoccur with discharge s well. I started him on l lysine 2x day for possibly 6 Mos.. I am happy to say that he has recovered completely and is symptom free since then. I know that l lysine does work.

  12. My pound Cat has been on L-Lysine since she was a kitten and diagnosed with Feline Herpes, at that time both her normal Vet and her Specialty Vet had advised us to use it. As long as she is on it her eyes are bright and clear when we forget to order more and run out her eyes start getting goopy. After reading some of these reply’s I will keep my cat on L-Lysine because I personally see the positive results.

    • Vetericyn Plus Antimicrobial Ophthalmic Gel for Pets.
      For my little old lady cat helps with the gooey goo goo eyes.

    • As a holistic healer there is another supplement you can give to your cat and I’ve done the research and vets even use it which I’m very surprised it’s not even mentioned here and it contains another amino acid called n- acetylcysteine a.k.a. NAC. It helps prevent secondary infections because it helps with mucus and it protects the kidneys in humans and cats. People do not feed their cats enough wet food and they are supposed to have it everyday. And good quality food without grains in it also because grains can aggravate these conditions. Vets will prescribe it as a medication but it is an amino acid and they call it Mucomyst. And if people are worried about vitamins being assimilated they can give their animal melted coconut oil on a plate and that’s good too because it kills viruses and it’s anti macrobial. My cat developed the runny nose thing and it’s been going on for 10 days and I noticed she did improve after I started giving her these two supplements. We moved to a place out in the sticks and there’s a lot of animals running around our cabin and I had her outside one day and I watch her closely as she doesn’t leave the house and I think she could have picked it up from other animals i e raccoons, foxes, deer and feral cats. She never had this problem before. Also there’s something called Homeopet nose relief. That helps. The FDA is trying to ban NAC because it’s a supplement and the drug companies are influencing them NAC is good for so many things you wouldn’t believe it it’s a super antioxidant and it stops even cancer cells in their tracks from growing! The drug companies and all these vaccine companies are just trying to make money. So yeah I think that some of these quotes studies are biased. I’m just surprised that no one mentioned the NAC. But they conveniently drop the n and just call it acetylcysteine. It work thins the mucus down and another side effect is it will get any Mercury out of the cat’s body as too many cats eat too much tuna which can contain mercury and it builds up very quickly and their system and can affect them. They should be given plenty of water. I offer my cat water in a cup and she drinks it almost every time I offer it to her when she sleeps with me at bedtime.

  13. I’ve been using L Lysine (gel & treats) for 18 months on my rescue with herpes. His sneezing (prev 20-30 at a time) has almost stopped completely and he no longer sleeps the entire day away. If I miss a dose or two, here come those sneezes! His infections are much more controllable now.

    • I have two cat rescues that I use L-lysine for. Paste from the vet and chews from Global Pets and or Pet Value. Symptoms are under control while using the lysine. However, if I happen to run out or miss some doses, symptoms come back. One is worse than the other. I would really worry if I did not have this to give to them. I feed high quality wet food, and try to keep everything natural. My home is very green.

    • I’m glad to hear this, because my cat was sneezing so much he had nose bleeds and taking L-Lysine stopped these symptoms and I have recommended it to others.

  14. Thanks for sharing, a decrease in arginine is detrimental to a cat’s health. My vet also told me that he no longer suggests people use this for their kitties, as it’s not effective and may cause kidney distress.

    As for the folks ripping into scientists, most are very good people that are using the scientific method to back-up the claims that so many have been making without conclusive evidence. There are reviewers that scrutinize every word and method, so being one myself, I know the rigor these people go through, no need to bash them.

    Lysine does not work the same way in humans and lysine actually does help with HSVI in humans, but being that it depletes arginine in cats and causes kidney distress, there’s no way my guys will get it ever again.

    • “As for the folks ripping into scientists, most are very good people”

      How do you know?? Who funded the study that’s what I want to know too. Lysine always worked for my cat.

  15. It is disquieting to me that so many ” researchers” are turning to retrospective studies ( I suspect as an easier way to meet their professional publishing requirements) which often do little to enlighten…seldom add anything to the body of knowledge and, in general, cause a lot of consternation among people who were happily using products that they felt WERE helping for years. I would SO prefer that more time and effort were put into NEW studies of these ( purportedly) time tested disease aids to examine ANEW if the claims are born out…. One always has to wonder if the individuals who did these retrospectives were judicious in their choice of articles or had a ” bone to pick” themselves….. I have nothing positive or negative to say about L-Lysine… I use it currently with my 3 FeLV+ youngsters as part of a VERY comprehensive regime of supplements…. Hard to say if it is working because I would only know if I stopped….waited for symptoms and then started again…… Many of us use supplements simply to feel some sense of control….as though we are DOING something….hoping that MAYBE it will do some good… It would be, of course, nice to have more definitive information

    • Taking science seriously when researchers use sound methods to show that something doesn’t work doesn’t mean they are attacking you personally or your vet. They simply said what they found and how they found it. Arginine depletion is rough on cats and when “researchers” try to enlighten others, I promise they do it because they are concerned with folks using an amino acid as a cure all when it is doing more harm than good.
      Many vets are now advising clients to back off of lysine, as renal failure is a bit more devastating than sneezing.

      • Simple answer. Give the lysine … supplement with arginine (and other amino acids cats need) if not on a raw diet.

        Diet plays a BIG role in cats health.

  16. I have a cat with herpes and for years I’ve kept it under control with
    VETOQUINOL Enisyl-F Lysine Treats but they’ve changed the formula and he won’t touch it now. Apparently LOTS of other people are complaining about the same thing. I don’t see my vet til next week for annual exam and bloodwork. What can someone recommend in place of this? He loved the old formula. If I use powder, I’m afraid the other cats will get some and my DVM says NO other cats should have the Lysine. Thanks in advance for anyone’s recommendations.

  17. I have read this study and it DOES NOT prove that lysine is ineffective or harmful for cats. You have an obligation to your readers to get a professional to evaluate this study. And it needs to be someone who knows how to interpret this specific type of study. One of the main problems with this study is that it was a retrospective analysis of multiple previously published research studies. This type of study can never PROVE anything. Secondly, the authors included old studies of lysine use in humans, dogs, and laboratory test tubes. Really?!?!? By doing this, the authors have violated one of the main rules for conducting these types of studies that look back at what other researchers have previously published. In fact, this study was so poorly conducted that no conclusions can be drawn from it at all, let alone making recommendations about how to treat our kitties.

    • Hear, hear. Not convinced by “reviews” of results of old, poorly conducted tests. One thing that did stick out was that *maybe* it’s not so good for kittens (presumably since their immune system isn’t fully developed?). Also, stress levels seemed to have a negative impact on all levels & increased the clinical signs. But we knew that, didn’t we…

  18. I have an 11 year old cat with feline herpes. My old vet recommended l-lysine and I had him on it per her instructions. We moved and my new vet actually recommends Forti-Flora which is a feline probiotic. I also have a tube of Teramycin for his eyes when he has a flare up. Both products can be purchased online. (My vet had prescribed the Teramycin for him previously).

      • My cats would NEVER eat anything that touched the “Forti-Flora”. Lol, I still have most of the box. I was disappointed, as it had such good reviews.

        I have had good results with a high quality (I feel) probiotic powder I get on Amazon (they also sell it themselves) of like 23 strains of probiotics for cats in it. It’s a reasonable price at about $35 a container. It’s small, but last awhile, depending on how many cats you give it to.

        I think it is a good quality probiotic, and as a person who knows firsthand how important probiotics are (especially after being very ill with c-diff) a good probiotic supplement is a must for cats health!

        Everything happens in the gut. You need good gut balance to absorb nutrients and many other functions in the body.

        I have never tried lysine but am researching it now. When I find a good brand, I am going to try it. Also going to look into supplemental amino acids to give along with it.

        • The brand is Pet Ultimates Probiotics for Cats. I just checked on Amazon and it is out of stock. The last time it was out of stock, I went to their website and ordered it from them. Free shipping and about the same price. I really like this product but don’t know what the heck is with them that they cant keep it in stock!

  19. I have two cat rescues that I give l-lysine treats to daily, it keeps both of there symptoms under control. If I forget to buy the treats and they miss out, there symptoms come back. Now I’m concerned about the side effects.

  20. This study can say whatever it wants. I have a cat with Feline Herpes, every day I put 1/4 teaspoon Now brand Lysine in his breakfast and again at dinner. When I forget ( I rarely have) his eye gets blinky and the sneezing begins. I also had a cat who came to me FIV positive, he also had Herpes and his eye ran constantly he was chronically congested. That darn Lysine made a huge difference.

  21. Per my vet’s direction, I give my cat L-Lysine twice daily since he has FIV (but shows no symptoms). I guess I need to research this a bit more. Thanks!

  22. I’ve heard a lot of people who claimed to have success using L-Lysine. Even my vet recommended it for one of my cats who is always sick with respiratory infections. But he refused to have anything to do with it, so I stopped trying to give it to him.

  23. I use the L-lysine treats for my cat that has Herpes and it has helped him not have any outbreaks. Was using the paste for my cat that is prone to colds and it never did anything for him.

  24. Oh, great, I have a large container of it, I’ve been giving it to 2 foster cats who have reoccurring colds (they have already been on antibiotics). I see the study was about the herpes virus, but what about lessening the length of colds (like taking vitamin C or Cold ease for humans?

    • I’m not aware of any studies that L-Lysine reduces the length of colds in cats, and given the evidence that it actually increased severity and frequency of disease in some cats, I would not expect it to do any good.

      • My cat was prescribed Tobramycin Ophthalmic solution eye drops and L-Lysine chews for a runny discharge from both eyes. She is better since the treatment. Was the Lysine not necessary? I will share this with my vet. Thank you!!!

        • Our rescue has used Terramyacin Ointment for years and years for cats and kittens with eye infections or “drippy eyes”. However, some vets we have consulted suggested plain saline instead. However, that’s heard to get into their noses. Another suggested we get a product for children called “Little Noses”, too.

      • I too have a cat that had frequent conjunctivitis. Vet bills, antibiotics and eye drops were expensive time consuming and not very effective. I started using Eyebright tea for my own conjunctivitis and discovered it is used for horses…it is amazing, usually only takes a couple applications and clears it right up. I just make the tea and swab around the eyes and make sure to get a few drops right in the eye…it does sting a bit but works within a day or two always! Mother Nature’s got it covered 🙂

        • I started us L Lysine for my cats who all have kitty colds….within a few days they all started clearing up their mucus changed to clear and then all but my 17 year old were fine, she though was much better. I went away for just a week and they didn’t get the L Lysine and I came back to every one of them coughing up lots of mucus and runny eyes. So back to the L Lysine and a week later they are all improving again! Not Rocket Science in my opinion the results here for us anyway speaks for itself!

  25. Agree; it hasn’t helped my rescue kitty, who has the herpes infection, in over 2 years. I’m sharing the article w/my DVM. Thank you, Ingrid!

  26. Thanks for that Ingrid!! One of my AZ kids was prescribed l-lysine for cold symptoms a few years ago. I don’t really recall it making anything better, and thank goodness it didn’t do any harm that we know of. However, if it happened again, I would’ve just done as the doctor order had I not seen this. As always, much appreciated!!

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