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

Sarah J. Wooten, DVM, CVJGhanasyam Bey, DVM
Updated on September 24, 2024

Key takeaways:

  • Lysine (Enisyl-F, Felisyl, Incremin, Optixcare, Viralys) for cats is a nutritional supplement that is available over the counter.

  • Lysine can control symptoms associated with feline herpesvirus, including eye and upper respiratory infections in cats.

  • Some research suggests that lysine is not effective. Other research says it does ease symptoms of herpesvirus.

A cat is examined with a stethoscope in a veterinary clinic.
Natsarin Kanna/iStock via Getty Images Plus

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.

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Lysine for cats at a glance

Common names

Lysine (Enisyl-F, Felisyl, Optixcare, Viralys)

What it treats

Eye and upper respiratory infections caused by herpesvirus

How it comes (forms)

Powder, paste, tablet, chewable tablet, chews, flavored gel

Common side effects

Loss of appetite, soft stools

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.

The bottom line

Lysine is a nutritional supplement that may help manage the symptoms of herpesvirus in cats. Veterinarians may recommend lysine because it’s relatively inexpensive and has a low incidence of negative side effects. Give your cat lysine once or twice daily, depending on your veterinarian’s guidance. Your cat’s eye and upper respiratory symptoms may ease in a few days. But it may take several weeks to notice the full effects of lysine.

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Why trust our experts?

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.
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.
Ghanasyam Bey, DVM
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.
View All References (7)

International Cat Care. (2018). Feline HerpesVirus (FHV) infection.

Maggs, D. J., et al. (2003). Efficacy of oral supplementation with L-lysine in cats latently infected with feline herpesvirus. American Journal of Veterinary Research.

Niedziela, K. (2016). Lysine for FHV? Researchers say don’t bother. Veterinary Practice News.

Satchell, T. (2024). The ABCs of feline herpes. Canadian Animal Shelter & Community Medicine Association.

Stiles, J., et al. (2002). Effect of oral administration of L-lysine on conjunctivitis caused by feline herpesvirus in cats. American Journal of Veterinary Research.

Thomasy, S. M., et al. (2016). A review of antiviral drugs and other compounds with activity against feline herpesvirus type 1. Veterinary Ophthalmology.

Washizu, T. (2003). Diagnostic approach to hyperammonemic conditions in young dogs and cats. Veterinary Information Network.

GoodRx Health has strict sourcing policies and relies on primary sources such as medical organizations, governmental agencies, academic institutions, and peer-reviewed scientific journals. Learn more about how we ensure our content is accurate, thorough, and unbiased by reading our editorial guidelines.

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