Lysine and Cats: Why It is Not Recommended

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• Hi Rochelle,

  I tried Lysine a few times and it was not as effective as I expected based on all the recommendations to use it. Now I don't use it anymore. I stick to diluted apple cider vinegar on my cat's fur since that is actually anti-viral and works wonders. If it is too bad I get antibiotics from my vet.

  I also give my cats an immunity boosting herbal supplement that gets mixed into their wet food. I am always trying to figure out if I should be giving them extra vitamins, probiotics etc. Right now I am trying a mixture of very high quality cat food with all that mixed in with medium grade cat food and some extra supplements. The food has a mushroom extract that is supposed to work wonders for immunity...but it costs a fortune, hence the mixing it together.

  Thanks for this great information on Lysine for cats. I have to say however, Lysine does seem to take down a cold sore right away for me and my husband. I always assumed it is a different virus in cats however and just doesn't work on them the same way. In any case, it is great to get more information on Lysine in cats.
  Thanks!
  Jessica

  • Hi, Jessica!

    The human herpesvirus and the cat herpes viruses are different viruses, just similar to each other and the way they behave, I believe. Regarding the human herpes virus responding to lysine, all studies on it have been 'inconclusive' from my understanding. The studies that suggest it is effective have never been done in a controlled setting, so they can't prove it was lysine that helped it.

    The one study I wanted to share with you isn't working on pubmed right now, but the article about it on the Harvard Medical School site is useful too.

    That's not to say it doesn't work! They just have no evidence it does except for a couple of studies that weren't controlled. Other studies indicate it isn't effective at all. So more work needs to be done!

    But because of those studies that suggested lysine was effective on human herpes simplex virus is how it started getting recommended on the cat herpes virus. Crazy, right?

    Anyway, glad to see another person who does their research on cat supplements! Thanks, Jessica!

• I have read other research and posts that support your claim that lysine is ineffective for treating FHV in cats. I haven't seen any positive reviews for arginine supplements either. My vet isn't a big proponent of most animal supplements, he syas they just pee them out without any real affect. I think your recommendation of controlling a proper diet is the best course of action to address the issue. 

  • Thank you!

    • Rochelle,

      Sorry, but my experience is very much the opposite!!

      I adopted an orange tabby who developed a condition that could not be diagnosed. The condition was one eye was closed!!! My vet could not treat it and in exasperation suggested we could just remove the eye!!!! But he did refer me to a feline specialist in optical problems. HE didn't know what to do. At this juncture, I spent close to $900 in vet bills. As a last resort, I tried Lysine. It worked, curing the condition within 10 days!!! So I'm sorry. When I think I was tempted to remove this lovable feline's eye ( who already had his teeth removed when I adopted him), I cringe. So I say thank God for Lysine. You could be giving other cats and cats owners information that could actually hurt our furry pets.

      • Hi! Thanks for visiting. However, I'm not giving incorrect information. I've cited many studies, linking to them, and other veterinary articles that give evidence lysine had no effect or in one study, made symptoms worse. One anecdotal case of it 'curing' a mysterious eye problem doesn't mean lysine helped. It could have been anything as your cat could have been given other medication, could have had a severe eye irritation that cleared up as soon as the foreign body was removed, or any other such thing could have happened. Only in blind test studies where there is a control group (a group given a placebo so that you can compare the results of the study from the group given the lysine versus the ones given a placebo). Nearly all feline illnesses can get better or worse on their own with no interference from people, which is why they have a control group.

        I'm curious, Lysine is only a supplement. It's only suggested for the feline herpes virus which is an upper respiratory infection. It wouldn't work on an unknown eye condition even unless it was runny or swollen shut from the feline herpes virus, IF it worked as people claim. And no vet I know would remove an eye unless that eye was damaged irreparably and useless, making an eye removal an improvement for the CAT. No vet I know would remove an eye from an upper respiratory infection unless that eye had completely ruptured. Not even for an unknown cause.

        Now, whatever caused your cat's eye to close was never diagnosed. It could have actually been something that lysine actually helped, for whatever reason, and not a feline upper respiratory infection at all. IF that's what caused the condition to get better. If that's the case, my statements still stand. I don't recommend lysine for upper respiratory infections. That doesn't make lysine supplementation worthless. Maybe your cat was lysine deficient. I don't know. But nothing you said proves it works on feline upper respiratory infections or the feline herpes virus. Just that you think it helped your cat's mysterious eye problem.

        I'm more concerned with your vet. I suggest you get a new one if they were wanting to remove an eye for an undiagnosed issue for no good reason.

• Hi, I just finished reading some very good points here I think it's probably the same with pharmaceutical companies for humans they are more interested in making money than people's health.  That's probably why they recommend Lysine because of the financial element.   I don't have a cat myself but I do think this is some very good knowledge your sharing here that could help a lot of people.

  • Thank you!

  • Totally right! Just so you know, I'm 9. That's why there are spelling mistakes.

    • Hey Lexie: Your spelling is perfect - better than a LOT of grownups. Good for you for getting info on your own!

• I've heard a little bit about lysine in cats but to be honest nothing so comprehensive as this, so I appreciate your knowledge on the subject. Sometimes it can be difficult to care for cats especially if you have multiple cats. To be honest it seems sometimes that hospitals and vets are more of a business than a place of healing. It's straightforwardness such as your post here that will not only help people care for their cats, but also keep the hospitals and vets honest. You've convinced me that lysine is not a good choice for fighting the cat's FHV infection, and that a healthy diet is the best option. My cat friends will love your post, thanks!

  • Thank you!

    • Sorry to use the reply like this, but could not find where I could comment directly. This study tested all combinations of lactoferrin, lysine, and antibiotics. "The aim of the present study was to verify the efficacy of lysine alone and of the association lysine-lactoferrin against FHV-1 in cats. ...These data suggest that the association lysine-lactoferrin could be a first choice in FHV-1 infected cats for reducing clinical signs and viral shedding." https://www.researchgate.net/publication/287592386_Evaluation_of_lysine_and_lysine-lactoferrin_association_in_cats_infected_by_feline_herpesviras-1?enrichId=rgreq-97a6d05d23c7cf9d42adebebf8205c0d-XXX&enrichSource=Y292ZXJQYWdlOzI4NzU5MjM4NjtBUzozNzUyOTQwNTA0ODgzMjBAMTQ2NjQ4ODQ4MjIxMA%3D%3D&el=1_x_2&_esc=publicationCoverPdf

      • Unfortunately, that study does NOT have a control group.

        Here's the problem with the study. Obviously the group treated with Clavamox ALL got better. The OTHER 4 groups were all treated with some lysine type product. There was NOT a control group that was given NOTHING.

        All this study proves is that 100% of the cats in the Clavamox group got better, while lysine was LESS effective. They just 'phrased' it as lysine "COULD BE" be an effective first line of treatment for feline herpes and viral shedding. What? They don't prove anything except that Clavamox works better.

        See, we need a group that wasn't given ANY treatment to determine the percentage of cats that recover WITHOUT TREATMENT. Do you know how many cats recover from a herpes outbreak without treatment? That's what you want to determine. You don't want to prove Clavamox works better. You want to determine if Lysine works better than NO treatment.

        So don't prove that Clavamox works better. I want you to prove Lysine cats improved in better numbers than the control group of with a placebo or no treatment.

• So you're the Barn Cat Lady. I love ti. Do you live on a farm and how many cats do you have?

  My Mother's father had a successful dairy farm in the Stoney Rises of western Victoria, Australia. When I was a kid we usually got their once a year. I loved that place there were all kinds of animals including cats. ferrets rabbits chooks dogs cows and snakes. Carrying a length of fencing wire was a must to kill snakes, but I only ever saw a couple.

  I liked the way you explained lysine that was my first question. I knew a vegetarian couple while I was at university. Vegetarianism was almost a religion for them.  From time to time they would mention how their lyfe style didn't require the killing of animals. I always thought it was ironic that they had a cat which as you indicated needed protein that humans didn't. So the cat ate meat. Does a cat have to eat meat?

  I like what you said about diet. I agree the immune system requires an extensive range of minerals. And in addition to a good diet, we all need less stress.

  • Hi! I actually own 3 cats, I have another 3 cats who live in the barn I care for, AND I have 10 foster cats and kittens!!

    You are absolutely right that cats must have meat. They cannot survive on a vegan or vegetarian diet. If they have a cat, they would have to be buying meat. This is partially why I haven't gone vegan (although health is the main reason!). It seems a bit hypocritical to me if I went vegan to stop supporting the meat industry, then still purchase cat food.

    Thanks for your excellent comment!

    • Hi -
      I don't think it's hypocritical to care for a cat (or multiple cats) and be vegetarian or vegan. Cats are OBLIGATE carnivores--they MUST eat meat. That's how they're designed.
      Humans have a CHOICE. That's how we're designed. (Sometimes I think we have TOO MANY choices, and that's actually a design flaw!)
      But a vegetarian or vegan who chooses that diet for health reasons should understand that, for HEALTH REASONS, they have to feed their cat/s meat.
      A vegetarian or vegan who chooses that diet for non-health reasons should understand that, for HEALTH REASONS, they have to feed their cat/s meat. But they can choose WHAT KIND of meat product/s to give their cat. They can choose only organic, or only humanely raised, etc.
      And, of course, another reason we would feed our cat/s meat products--because we love them!

      • Hi! You're right that a cat needs meat and there's no choice about it!

        I believe I said I'd feel hypocritical and here's my reasoning.

        If you go vegan for health reasons or because you yourself can't eat animals, then that's perfectly fine when you own a cat, who has to eat meat in his or her food.

        If you go vegan because you are morally opposed to the meat industry for animal rights reasons or because of environmental reasons, but then adopt a cat, who forces you to buy cat food with meat in it, which in turn supports the very industry you are against. See what I mean? It's hypocritical. But if you're aware of the hypocrisy when you go vegan, then that's cool too!

        I can't go vegan for health reasons myself, but if I went vegan, it'd be because of the animals and the environment. Then I'd feel hypocritical. Vegan and vegetarian diets often require some supplementation and both diets have been linked to depression, although they aren't sure if the diet caused the depression or if depressed people are more likely to go plant-based. I simply chose the least cruel options I can, eat as little meat as possible, and supplement what my doctor recommends. But at least I don't feel badly about having cats who need to eat meat!

        I'll have to double-check to see if I explained that as well as I did here.

        Thanks for stopping by!

        • Greetings!
          I'm a vegetarian and I make raw food for my two cats. I grind the meat myself (16 pounds every two weeks, they're Maine Coons) and include a nutritional completer, heart, liver and salmon oil. I don't feel conflicted or hypocritical at all because cats are obligate carnivores and will die without meat. For me, I can live healthy without meat. Hope this helps anyone thinking about feeding meat to pets and their humans being vegetarians. :-)

        • I just clarified my comment up there. I didn't mean that all vegans or vegetarians are hypocritical. But the gentleman mentioned they were treating veganism as a religion and how their lifestyle doesn't depend upon the killing of animals, but they have a cat... THAT is hypocritical. If someone doesn't want to eat meat or kill animals to consume themselves, but love cats and understand they need meat, awesome. I'm mostly veggie myself, but I have a horrible vitamin deficiency on a regular diet so already have to supplement already and my doctor vetoed vegetarianism or veganism for me completely unless I go veggie with fish (which I don't like eating fish, so that's completely not an option). I have no choice. If I went completely veggie, I'd do it so no animal had to die for me to eat, but I'd understand that by choosing to have a cat, I was still supporting the killing of animals for food. Just supporting the killing of them for cat food. But they definitely can't run around bragging about their lifestyle not hurting any animals if they have an animal they have to feed meat.

          That's all I meant by that particular comment. It's for those that are condemning people as immoral for being unable to go vegan, while at the same time, they have to buy meat for their cats.

          Or they try turning their cat vegan, which isn't good.

      • Right on Colby people will find any reason that they can NOT TO BE VEGAN then call others the hypocrites.

        • I didn't call vegans hypocrites. I said "I" felt like a hypocrite because I save cats and was forced to support meat production because I love cats, etc. And it's responses like this is why I don't like speaking to some vegans.

          I'm vegetarian, BTW. And I actually failed at being vegan due to health issues as I got seriously ill when I tried after about a year. Once I started adding dairy and eggs, I did much better. Not EVERYONE can go vegan, as nice it would be. There are actual genetic differences where some people can properly get nutrients out of an all-plant diet and some people cannot, because these people lack certain enzymes needed to break down certain nutrients into other nutrients. It's like being lactose intolerant. Some humans can break down lactose easily, despite not being infants. It's an evolutionary thing humans started developing after they started drinking cow milk. But not ALL humans can tolerate lactose after they're weaned from mom.

          Some humans can, and some cannot. And my doctor and I learned I did not do well, even WITH a ton of supplements.

          But I did not say vegans were hypocrites. I said that *I* FELT like a hypocrite. Sigh. Many vegans suffer pangs because they feed their cats meat. It is NOT just me.

• Perhaps the science does not know why it helps in some cats (note: obviously not ALL cats, based on what virus is causing the URI or your cat's individual immune system) -- evidence anedotally supports that for many cats it does help. Science is always evolving and changing based on new discoveries. Working in a vet office I can say many cats benifit greatly from Lysine, clearing up URI. Two of my cats in fact could not do without it -- within a week they will have sever eye issues and will be sneezing out awful snot if they do not have it. Why does it work? No idea. But they need it.

  • You're right. Science and evidence is always evolving based on new evidence all the time. The problem arises from doing studies on cats on lysine versus cats on a placebo. Now cats can't be fooled by the placebo effect, but a lot of the time, URIs will self resolve on their own as they're viruses and require the cat's own immune system to fight off. So we can't know if the anecdotal evidence is actually due to the lysine or due to the cat's own immune system. Some cats will definitely get better on it, simply because they would have gotten better without it. That's why anecdotal evidence isn't actually considered proof. Especially as the studies I've seen have all indicated that lysine performs on par with the placebo test, meaning no increase in resolving symptoms. Admittedly, there's only one study that suggests it made symptoms worse. AND they're studying lysine on feline herpes, not on every other type of URI there is out there. And scientific studies on cats are lacking in a lot of ways,so it may be we will find out lysine works on in some unseen way on certain things.

    It's like ringworm and some rescues and vets are suggesting that the treatment isn't actually working any faster than a cat who isn't treated since ringworm almost always self-resolves in cats, no treatment required. Which is a good thing with the barn cats, I can't imagine trying to bathe them in lime sulfur dips or with anti-fungal shampoo. But there seems to be heavy evidence that treatment is not any more effective and it's simply the cat's immune system.

    We don't know. I know just as many people who agree it didn't work as swear that it did, and the studies I've found agree that it isn't any more effective than placebo, so that's why I don't use it.

    It is up to each individual and their vet to decide for themselves to use it or not to use it, of course.

• We adopted a kitten that was diagnosed as having FHV or a URI. She would sneeze awful snot rockets, and her left eye would get all gunky. I tried lysine and another immune booster I read about (forget now what it was) and neither seemed to help much.
  However, I went back to the lysine (I forget why now) and have been giving it to her in her nightly wet food snack and it has since cleared up (for about the past year). She's become a very picky eater and for awhile I was having a hard time getting her to eat it on a daily basis & her eye did indeed get worse when she wasn't taking it.
  We have a big 22# long-haired cat (Norwegian Forest cat we think) who often has problems with diarrhea (which makes it so hard for him to stay on top of grooming himself - it's gross). I've tried getting him to eat pumpkin, and coconut oil, to help supplement his diet (didn't work - he wouldn't eat it). Got the idea to also feed him the lysine if it's supposed to enhance immune support. So he too has been eating it for the past 6-9 months and has not had ANY diarrhea issues! This has been a godsend!
  I just got online to see if they can eat it daily, or if now that their problems have cleared up, I should maybe be dialing it back a bit to maybe only 4 or 5 days a week... As I know some things you don't want to take long term.

  • Ah, but was it the lysine that helped or was it something else? We can never know if something self-resolved on their own, the lysine did something, or if it was a different thing we tried. That's why I always look at scientific studies. They compare the results of giving lysine to one group and giving a placebo to another group. The placebo group is the control group. Some animals will improve in the placebo group, but if something works, it needs to do better than the placebo group. Lysine doesn't perform better than the placebo.

    That said, I have no idea if it is a good idea long-term or not. There actually isn't a lot of research on it in cats, except the two or three studies that indicate it doesn't work any better than placebo/no treatment. In one study, it even made symptoms worse. But there isn't a study on long-term effects that I've found.

• Thank you for this new-to-me angle on lysine. I was about to give it to my kitty but now I'm going to read more and consider carefully. I do feel obliged to comment on two things: I have had absolutely miraculous results using lysine with my own cold sores (I'm a human, btw), and I astonished my dermatologist with the amazing improvement it accomplished in my shingles. Now he recommends it. It's inexpensive and harmless for humans at normal doses for a short time, so I truly believe it's worth trying with any human herpes problem. Secondly, I have been vegan for a several years, vegetarian for almost fifty, and I am not at all depressed or unhealthy, nor are any of the many vegans I know. My cardiologist is blown away at my perfect blood pressure at age 70. I wish I could avoid supporting the meat industry in any way, but having rescued many obligate carnivores in my life (and think of them as my closest friends) I don't feel I have a choice. I don't feel that this is hypocrisy, just an unfortunate necessity. I would love to see a pet food company produce a food line that is not only organic but as humanely produced as possible instead of factory-farmed. Thanks!

  • Hi, Sandy,

    I have no idea if lysine would help with Shingles issues or not, that's one thing I didn't research. But here's the problem with anecdotal evidence. It's not done in a controlled study where one group of people/cats is monitored taking ONLY lysine for herpes symptoms while another group is given a fake lysine. Then they compare the two groups. Because even in the FAKE group, some people WILL get better. But does taking real lysine actually IMPROVE the results more than the fake group?

    In anectotal evidence as when people swear something happens, they tend to associate one thing with getting better. But we don't know if it was THAT that helped, the body's immune system, or even something else they took finally started working. We have no idea. And in people, people can be fooled by the placebo effect. Basically, if someone THINKS they are getting real medicine, they actually FEEL better, even if they were given sugar pills. That's why they test improvement rates against placebo groups. It helps create a baseline on the average number of cats/people who get symptom improvement with NO medicine versus a group given a medicine. Did more people get better then? Or did the same number of people get better? So I put little stock in people's experience with it. I'm always reminded about people who SWEAR they got the flu after the flu shot. What's funny is most of the time, they got sick before the flu shot was even given and they experienced symptoms immediately after the shot. But did the flu vaccine actually give them the flu? Not according to all known vaccine studies. Not a single person caught the flu from the flu shot during clinical trials. But some people SWEAR it happens.

    It's why I look at the percentage of people who improve with a medicine vs. the group that weren't given the treatment. Because some cats/people will improve with NO treatment at all and we can't say for sure whether it was lysine or the immune system that actually helped.

    As for the vegan thing, that's just how I feel. I also have severe deficiencies on some things even ON a normal diet and have to carefully supplement as it is. My doctor warned me not to go vegan. Some people absolutely CAN and do thrive that way. I just can't. I do however try using more humane options than factory farming. Free-range chicken eggs and I've limited my meat. And many cat food companies will do the same, but it can get pricey. And when you care for dozens of cats, they might not be able to afford ethical cat food. So it's a toss-up. *I* just feel a little hypocritical that I feed my cats meat by necessity and can't afford more humane options for them without going completely broke and that I myself can't thrive on a vegan diet. But anyone that tries reducing the cruelty of their food intake is helping, too, whether by going vegan or even just free-range. The meat industry IS changing thanks to all of that.

• I didn’t know about L-Lysine, until my veterinarian told me, to try, as an adding factor for a serious case I had with a cat of mine.

  But look, that was as a plus only. She said, well you can also try to give some L-Lysine, it may help.

  The cat was found in the neighborhood near to death, dehydrated, with serious weight loss, and serious rhino sinusitis, actually nose and lung infection. Nose liquids, not eating etc.

  That was not FHV. The vet told me as a sinus virus with companion bacteria. We made antibiotic injections (I think 3 times every 2 days), Vet told me that they were Amoxycillin, she also told me to continue with Amoxycillin by mouth (Augmentin), or alternatively with Doxycycline (Vibramycin).

  We continued, the cat started to eat, and get weight every day. But the sinusitis was continuing also.

  We even got lung X-rays, and the X-Ray lab told me that sinusitis may go a chronical problem for life.

  May the inner of the nose, had some distruction by the long time infection.

  I lost my faith, using Augmentin (1 week) or Vibramycin (1 week), for a long time (15 days), until the Vet told me again to insist with Amoxycilline for up to 25 days.

  You may also use some L-Lysine in the food to help, she said.

  We went for the antibiotic cycle for 25 days using an other antibiotic, Amoxil, calculating the doses per kilograms of body weight, and the 12 hour cycle of the doses, with integrity.

  So, the miracle happened, and that so difficult sinusitis vanished!

  To say the truth, I also help the cat with good nutrition and vitamins. So, I can not conclude if the L-Lysine helped or not. But I’m sure my success was the long time (25 days) of the antibiotic cycle, and that for me and my cat, Amoxicillin finally worked.

  Today, it’s sinusitis is gone, but it left some little remain, I hear it, when he sleeps and snores.

  I had many FHV cases with other cats. The key factor is time. Time is critical, to save the eyes, and start antibiotics immediately, also antibiotic eye drops / antibiotic eye cream, we use Tobrex (Tobramycine) as an eye drop antibiotic.

  Or if you have a serious case, you go to the vet for antibiotic injections as the start of the therapy, and you continue antibiotics by mouth, and eye drops.

  The tragedy is always, if you lost time, and you lost an eye, or the worst, both of the eyes. But in some cases, the cat can be saved with a medium eye damage, but she will have vision after all.

  My conclusion is: L-Lysine may add some help (?) but the basic is the antibiotics, both for the body, and for the eyes (drops).

  And of course, enforcing the cat with nutrition, vitamins, fish oil etc.

  • Absolutely! L-lysine is often used instead of medication. Of course, as long as proper meds are used as well, it's not harmful and 'may' help. Just at this time, there's no evidence it helps the feline herpesvirus, but that doesn't mean it isn't beneficial or effective for other issues, just they haven't studied it.

• I have a cat that came from the shelter with some sort of URI, his eye got gunky and he sneezed bad. The vet said it was FHV by never ran tests so take that with a grain of salt. They sent me home with a l-lysine paste to give him and he did get better. I agree he might have gotten better on his own. However, fast forward in time and he had flare ups. I didn't believe the l-lysine helped, but I finally felt bad for the guy. He be miserable for a few weeks every time he has a flare so I bought some. Wow! He cleared up in one to two days of his first dose of the stuff, and if I stopped giving it to him too far the flare up comes back. I have no idea why it works or if he even has FHV for real, but it does work for him, and I used it on a pair of feral born kittens in my yard and they cleared up too. Take the info or leave it, but I'm going to use it because I can't stand listening to my poor guy sneeze, shake his head, and mope around miserable. The vet also recommended having him get shots for FHV regularly but he is so traumatized by going to the vet and the l-lysine world better than more shots ever did.

  • Hi, Dawn,

    Some people swear by L-Lysine. It did nothing to help mine. So I decided to research studies on it. That's when I realized ALL the studies they've done show it performs no better than the control group who is on a placebo! In humans or cats! That's when I made my decision.

    But here's my other problem with L-Lysine supplements. It was recommended for FHV only, right? (And yes, your vet diagnosing without doing lab tests to determine if he or she is right isn't very cool. MY vet doesn't recommend those tests all the time either, but he calls all of those types of illnesses, upper respiratory infections or URIs. He doesn't suggest WHICH type of URI it is without doing lab work. He usually just treats the secondary infection with antibiotics and waits for the cat's immune system to fight off the virus. If that doesn't work, THEN he suggests those tests.) But then these people decided to make MONEY off of L-Lysine and started marketing it for IMMUNE support, instead of marketing for what doctors were hypothesizing it would do for the herpesviruses. The ONLY studies I've seen that support L-Lysine usage is for those people who are lysine deficient! Everything else is a "eh, maybe?"

    Here's my other problem: supplements are not regulated by the FDA or any agency until AFTER a complaint has been made and the FDA tests the product to be sure the product doesn't contain actual drugs, prescription or otherwise, or other 'hidden' ingredients not included on the label. So who's to say the supplement company didn't add something extra to that lysine, like antibiotics or antiviral medications, to help it fight off severe URI symptoms. We don't know. By law, they're not supposed to. But there are hundreds of supplements that have been pulled off the shelves by the FDA after it was found to contain prescription drugs, illegal drugs, or other problematic ingredients without proper labeling. You think I'm kidding, but I'm really not. Male enhancement and weight loss products are the two most common 'supplements' that companies will include prescription drugs in them to make them actually work JUST to make money. I do a LOT of research on supplement companies before I choose one for myself, let alone my pet.

    But yeah, it may have worked for you and maybe it was legitimately lysine, and maybe it was the cat's immune system as well as any other meds the vet prescribed. Anecdotal evidence gives us hints onto things to study, but it's not accepted as proof scientifically. By all means, keep using it if it's working. Maybe your cat is just lysine deficient and that's why it helps! In which case, great.

    But it didn't work for me and all studies I could find testing lysine on FHV indicate it doesn't perform any better than a placebo. Add in my irritation on their marketing practices and lack of trust in companies who deal in supplements, I still don't recommend it. ^_^