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Lysine and Cats: Why It is Not Recommended

Lysine and Cats: Why It is Not Recommended in Cats with Feline Herpes Virus (FHV)I get asked a lot about lysine supplements for upper respiratory infections in cats.  Lysine and cats seem to come up quite often when discussing a sick cat, in a community cat colony or a home!

Many cat caretakers, shelters, sanctuaries, and vets have recommended lysine supplementation in cats who have Feline Herpes Virus (FHV), a type of feline upper respiratory infection.

I do NOT recommend it for the use of controlling FHV. Why? Because it doesn’t actually work and evidence suggests it makes things worse! Read on for my reasoning!

Disclaimer: I am not a veterinarian and this does not constitute veterinarian advice. Please contact your veterinarian for any health concerns you have for your cats. I simply use my experience and research to help educate cat lovers, feral cat caretakers, and rescuers.

Disclaimer:  This post contains affiliate links.  This simply means I will earn a small commission on qualifying sales at no cost to you.  Read Affiliate Disclosure for full details.  Thanks for supporting the kitties!

What is Lysine?

Lysine is an important amino acid that must be obtained through diet as neither your body nor your cat’s body will make its own lysine. It is used the biosynthesis in proteins. It is also important for normal growth and muscle turnover, and to build carnitine, a substance found in most of your cells!

Now, I’m not going to go TOO technical here, but a quick rundown of what it is might be useful for you.

A lack of lysine can also cause some health issues in humans (and cats!) like defective connective tissues, impaired fatty acid metabolism, and more. Too much lysine, caused by ineffective catabolism, and you can have neurological issues.

It’s a biological thing.

For the purpose of supplementing lysine to help with Feline Herpes Virus (as well as human’s herpes simplex virus (HSV)), it is SUPPOSED to work by limiting arginine, another amino acid. HSV (human herpes) is actually richer in arginine and poorer in lysine, and thus lysine supplementation was tried to help control HSV and later, FHV (feline herpes).

Let’s state this plainly:  Lysine is NOT a supplement for immune support.  It is supposed to be a supplement to fight off the feline herpes virus by reducing arginine.

To be clear, human herpes and feline herpes are NOT the same virus. But they are similar to each other.

Now arginine, in humans, is considered a semi-essential amino acid and can be synthesized by your healthy body. No need to supplement it.

However, cats CANNOT synthesize their own arginine. They MUST get this amino acid through their diet. It is considered an essential amino acid for them. It has been suggested that using lysine to reduce arginine (if it works) can be very dangerous for our feline friends.

Cats are HIGHLY sensitive to arginine deficiency! Arginine is very important to detoxify ammonia, which results from the breakdown of proteins. It also has an important function in the urea cycle. According to WikiVet, “feeding a diet that has no arginine but contains substantial levels of other amino acids can result in death within a few hours.”

In kittens, arginine deficiency can cause diarrhea, weight loss, food refusal, urinary issues, and more!

Tiny Orange Tabby Kitten Face

Does Lysine Actually Reduce Arginine?

Lysine does NOT have antiviral properties and will not improve a cat’s immune system, as I stated previously.  Even if it were effective on the feline herpes virus, it would not work on the various caliciviruses or feline chlamydia or other upper respiratory infection agents.

To put it simply, lysine and arginine use the same intracellular transporter system and thus too much of one may affect the metabolism of the other. Notice that I said MAY affect the other. It is a theory.

So far, I have not found any studies that actually state the arginine is reduced in medical studies. The levels of lysine in the blood increase, but that doesn’t mean levels of arginine decrease throughout the body.

Feline Herpes Virus (FHV)Sick Tabby Cat with Feline Herpes Virus

The feline herpes virus is one of two types of viruses that cause most of the upper respiratory infections in cats. It is not as common as calicivirus, but it does account for a large percentage.

Read more about feline upper respiratory infections here.

FHV and other upper respiratory infections cause sneezing, congestion, runny or inflamed eyes, coughing, or other symptoms like fever, pneumonia and in some cases, death.

What separates FHV from calicivirus is that once a cat has contracted FHV, that cat will be infected for life and have occasional reoccurrence of upper respiratory symptoms. Just like human herpes, sometimes the cat shows symptoms, sometimes they don’t, basically. They are also infected for their entire life and occasionally shedding the virus, thus are contagious to other cats.

Calicivirus on the other hand usually gets the cat sick one time, though they may end up infected for months, years, or their entire life.  Sometimes they may even shed the calicivirus and thus be contagious to other cats.

Just like other herpes viruses, the feline herpes virus is high in arginine. Thus, the theory that supplementing with lysine will reduce arginine and subsequently reduce herpes symptoms, how frequently it happens, and virus shedding, which can infect other cats.

Studies on Lysine and Cats

There are numerous studies that have actually proved that lysine supplementation does NOT prevent or help treat feline herpes virus.

Some studies have actually reported an INCREASE frequency of FHV episodes and virus shedding when cats are supplemented with lysine.

In case you’re wondering, it also not effective for preventing cold sores or other herpes symptoms in humans either!

There are many studies published that have proved that lysine supplementation is completely ineffective in FHV cats and/or INCREASED virus shedding and symptoms.

Here are a couple:

BMC Veterinary Research: Systematic Review

PubMed: Study of Effects of Dietary Lysine Supplementation on Infectious Organisms in Cats in a Shelter

PubMed: Effectiveness of Dietary Lysine Supplement in Cats with Enzootic Upper Respiratory Disease

In fact, only ONE study noted that it helped cats with conjunctivitis symptoms from the herpes virus, but that was a seriously small study of EIGHT (8) cats. That’s 4 cats treated with lysine and 4 cats in the control group to compare the two.

There’s a veterinarian article discussing both points of view about lysine, so you can make your own decision.

Though, if you’re still interested in trying lysine supplements for your cats, you can find some on Amazon or Chewy.

Viruses in a Lab

Do I Recommend Lysine Supplementation for FHV? No

Lysine has no antiviral properties. It cannot even be proven it limits arginine. If it DOES limit arginine, that could be dangerous in cats, as they do not produce their own. There have been studies of it proving it actually made symptoms and viral shedding worse!

The only reason lysine was recommended in the first place was based on a HUMAN study on the herpes simplex virus. And just to say, there is no evidence it reduces cold sores either.

I know vets, shelters, and rescues do recommend it. I don’t.

So no, I don’t use lysine supplements nor do I recommend them for people caring for cats with upper respiratory infections.

What Does Help a Cat’s Immune System?

Let’s be clear, lysine was only suggested for use in feline herpes virus NOT immune system response, despite the numerous products that claim it helps a cat’s immune systems.

The most important thing you can do to help a cat’s immune system is diet, to be honest! The better the nutrients a cat ingests, the better the cat’s overall health is, including the immune system.

In my outdoor cat food post, I recommend the best food you can afford for outdoor cats.  This is still true.  Bad food is still better than NO food, definitely.  That being said, the better the food, the healthier the cats!  So my first recommendation is to save the money you spend on lysine supplements and buy your cats better food!

Another suggestion is probiotics!  There has been some early evidence that probiotics help your overall immune system, not just your gut!  It is suggested that it does the exact same thing in cats!

Another suggestion is that arginine supplements actually INCREASE immune system responses in cats!  That’s the same amino acid that lysine is supposed to reduce!  (Though no evidence shows it does reduce arginine in cats). Another possibility is supplementing with salmon oil as the results are promising for that as well.

My last suggestion for boosting a cat’s immune system is reducing stress!  Stress is a key factor in the reoccurrence of the feline herpes virus as well!

Have you used lysine for FHV? Do you think it helped? Or not? Leave me a comment!


21 thoughts on “Lysine and Cats: Why It is Not Recommended”

  1. 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.

    1. 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!

  2. 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. 

      1. 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.

        1. 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.

  3. 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.

  4. 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!

  5. 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.

    1. 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!

      1. 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!

        1. 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!

          1. 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. 🙂

          2. 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.

  6. 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.

    1. 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.

  7. 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.

    1. 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.

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