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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!


10 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. 

  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. Seems a bit hypocritical, right? I don’t eat a lot of meat, but I am not fully vegetarian or vegan.

      Thanks for your excellent comment!

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