Lysine for Cats: A Supplement for Some Eye and Upper Respiratory Infections

Lysine (Enisyl-F, Felisyl, Incremin, Optixcare, Viralys) is a natural supplement that manages symptoms of feline herpesvirus (FHV). FHV is a highly contagious, common viral infection in cats. Its symptoms include eye and upper respiratory infections.

Lysine has few negative side effects and is relatively inexpensive. This makes it worthwhile for pet parents to see if it manages their cat’s symptoms.

In this article, we will discuss what lysine is, how it works in cats, and common side effects.

Lysine for cats at a glance

What is lysine for cats, and how does it work?

Lysine is an amino acid and a nutritional supplement that helps reduce FHV symptoms, such as eye infections in cats.

We don’t know exactly how lysine works to reduce symptoms. But it seems to interfere with arginine, another amino acid. Without lysine to counteract it, arginine reproduces the virus in cats. Keep in mind, your cat needs the right amount of arginine to stay healthy.

Lysine is sold over the counter and does not require a prescription. It’s considered off-label because it’s not approved by the FDA for use in cats.

What is lysine used for in cats?

Lysine is a dietary supplement that may help manage FHV symptoms. Herpesvirus is a highly contagious, incurable virus that is common in cats. The virus’ symptoms include:

• Painful conjunctivitis that causes squinting

• Eye discharge

• Redness in the eyes

• Loss of vision (in severe cases)

FHV can also cause upper respiratory infections with sneezing, nasal discharge, and in rare cases, skin ulcers.

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Studies on the effectiveness of lysine to manage FHV symptoms showed mixed results. Some studies concluded that lysine is effective, but many other studies suggested it’s not. Most of the evidence to support the use of lysine in cats is based on pet owners who say it helps.

How is lysine dosed and given to cats?

The standard dosage of lysine for cats is 400 mg by mouth once daily mixed in with a small amount of food. But a vet may recommend higher dosages given more often.

Lysine is available over the counter, though many veterinarians keep it in stock at their clinics. It comes in several oral forms, including:

• A powder that can be mixed with food

• Crushable tablets

• Chewable tablets

• Paste

• Gel

Lysine typically takes a few weeks to reach its full effects. But you may notice gradual improvements in your cat’s symptoms after a few days.

What are the common side effects of lysine in cats?

Research is mixed on potential side effects. Some research suggests cats may experience gastrointestinal side effects, including reduced appetite and soft stool. If side effects occur, give lysine with food. Some cats may throw up if they swallow a large amount of lysine.

You should give your cat lysine only if they tolerate it. If giving your cat lysine causes stress or loss of appetite, stop the medication and consult your veterinarian.

What are the most serious risks of lysine for cats?

There isn’t enough research to fully understand the serious risks of lysine for cats. But researchers point out that lowering a cat’s arginine levels too much can cause problems.

Arginine is an important amino acid for your cat’s well-being. Because lysine may lower arginine, it could cause a condition called hyperammonemia, which is when there’s too much ammonia in your cat’s blood.

Symptoms of this life-threatening condition can include:

• Lethargy

• Circling

• Head pressing

• Blindness

• Seizures

• Coma

Does lysine interact with other cat medications?

Lysine has very few drug interactions. Use caution when the following medications are given at the same time as lysine:

• Arginine (may interfere with lysine absorption)

• Calcium supplements

Be sure to tell your veterinarian about all medications, vitamins, and supplements that your cat takes before giving them lysine.

What should you do if your cat misses a dose of lysine?

If you miss giving your cat a dose of lysine, give the next dose as soon as you remember. Then resume the regular dosing schedule the next day. If it is close to the time of the next dose, skip the missed dose and give the next dose at the regular time. Never give 2 doses at the same time to catch up.

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Written by:

Sarah J. Wooten, DVM, CVJ

Dr. Sarah Wooten, DVM, CVJ, is a well-known international influencer in the veterinary and animal healthcare spaces. She has 16 years of experience in private practice and more than 15 years experience in veterinary media work.

Edited by:

Renée Fabian, MA

Renée Fabian is the senior pet health editor at GoodRx. She’s worked for nearly 10 years as a journalist and editor across a wide range of health and well-being topics.

Reviewed by:

Ghanasyam Bey, DVM

Ghanasyam Bey, DVM, is from Cleveland, Ohio, and attended Princeton University for undergraduate studies. After a year of biology research at Duke University, he attended Ohio State College of Veterinary Medicine.

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