L-lysine For Cats: Overview, Dosage & Side Effects

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L-lysine, or lysine for short, is an amino acid supplement. It is often used in cats for immune support with the intent of curbing or suppressing the effects of the feline herpes virus (FHV). In this article, you’ll learn what lysine is, how it helps cats, potential side effects, and some frequently asked questions.

Quick Overview: L-lysine For Cats

Medication Type:
Amino acid supplement
Oral gel, oral powder, oral chews.
Prescription Required?:
FDA Approved?:
Life Stage:
Most lysine supplement products are labeled for use in kittens.
Brand Names:
Viralys, Enisyl, Felisyl, Optixcare.
Common Names:
Available Dosages:
Gel: 250mg per 1.25ml; Powder: 250mg per scoop; Chews: 50mg per chew.
Expiration Range:
Lysine should be stored at room temperature. Products should be used before the expiration on the package.

About L-lysine For Cats

human hand holding supplement in front of cat

Cats that lead stressful lives, such as cats living in rescue shelters, are more susceptible to FHV.

Lysine is an amino acid. Amino acids are the most basic components of proteins and are needed for many processes within the body. These processes include the creation of new proteins, hormones, and neurotransmitters. There are a total of 20 amino acids, with some being essential and others considered non-essential.

Lysine is considered one of the essential amino acids, along with tryptophan, methionine, and a few others. Examples of some non-essential amino acids include alanine, glutamine, and arginine.

The basis of lysine’s role in helping with feline herpesvirus is that it competes with arginine in its role with herpes viruses. Arginine is believed to be required for producing infective viral particles that allow a herpes virus to infect a host. When lysine is incorporated into the virus and outcompetes arginine, the virus is then thought to become less infective. 

Also Read: Feline Viral Rhinotracheitis (AKA FHV): Causes, Symptoms, & Treatment

What Does L-lysine Do For Cats?

multi-cat sleeping and looking at the camera

Very young kittens are often more susceptible to FHV.

Feline herpesvirus (FHV) and specifically feline herpesvirus-1 (FHV-1) is an extremely common virus that infects cats. It is especially common in very young kittens, cats with poor immune systems, and cats under a lot of stress, such as those in a colony or shelter environment. 

Feline herpesvirus can cause conjunctivitis (eye inflammation) as well as upper respiratory signs of sneezing and nasal congestion in cats. A large majority of cats presenting with signs of sneezing and eye discharge have underlying herpesvirus infection.

The problem with herpesvirus is that while symptoms may come and go, it can never truly be removed from the body. Cats that have herpesvirus may continue to have flare-ups of conjunctivitis and sneezing throughout their lives. Flare-ups occur more often during times of stress, like during a move to a new home or the introduction of a new baby or pet that changes their familiar routine.

Feline herpesvirus can also predispose cats to secondary bacterial infections that can lead to worse signs like fever, heavy nasal congestion and inability to smell, bacterial conjunctivitis, lethargy, and poor appetite.

While improvements have certainly come along in the last 20 years, antiviral drugs generally have been more challenging to develop. This is because there are fewer parts of a virus for a drug to target than there are for bacteria. In addition, antivirals available, unlike most antibiotics, cannot cure a viral infection. Instead, these conditions require life-long therapy. 

These challenges led to an interest in L-lysine’s potential to reduce the effects of herpesvirus symptoms through the administration of what is essentially a nutritional supplement that has very few perceived side effects.

Despite its promise, L-lysine has a varying record in research studies. The use of lysine has been shown to suppress the shedding of the virus and to reduce the severity of conjunctivitis in experimental conditions. 

However, at least in shelter cats, it has not been shown to significantly improve or prevent upper respiratory tract infections. Shelter cats are predisposed to a lot of stressors and potential infectious organisms, which can complicate treatment monitoring and outcomes.

L-lysine is still recommended by many veterinarians for use in household cats with a history of herpes virus. Administering it to cats in the comfort of their own home may be less stressful and there may be fewer stressors complicating their health.

Also Read: What Is the Difference Between a Stray Cat and a Feral Cat?

Side Effects Of L-lysine For Cats

cat shedding and cat hair in cat brush

Fortunately, side effects of L-lysine are rare. Adverse effects have not been observed in clinical trials. Overdose toxicity is also considered unlikely. However, gastrointestinal effects like decreased appetite, vomiting, and diarrhea can occur if a cat ingests a large amount of lysine.  

Also Read: 15 Best Cat Foods For Shedding

L-lysine Dose For Cats

cat taking supplement

Cats may take L-lysine as a snack or treat, and it’s recommended that they consume it in one sitting.

There are many products containing L-lysine with the intention of being used as a supplement for cats. While many products may be labeled for use in cats and kittens, it is important to note that no products are FDA approved for treating or curing cats of herpesvirus, and all doses are considered extra-label.

  • L-lysine gel (Viralys is a common brand but there are many generics) comes in 250mg per 1.25ml. 
  • L-lysine powder (Viralys, again, is a common brand with generics available) comes in 250mg per rounded scoop.
  • L-lysine chews under the brand Optixcare have 500mg per soft chew treat. 

The recommended off-label dosage for L-lysine in cats using one of these types of products is 205mg for a kitten or up to 500mg for an adult cat. Doses can be given once or twice a day. 

Generally, it is recommended that a cat consume the entire dose in one sitting. For example, a kitty should consume all of the gel or chew it as a treat. If the powder is used and mixed with food, it should be eaten all at once and not snacked on throughout the day. 

Administration of L-lysine should be a pleasant experience for your cat. The stress caused by forced administration is thought to possibly counteract any potential benefits. 

Also Read: The 7 Best Cat Vitamins And Supplements


human brushing cat fur

House cats that take L-lysine in the comfort of their own home are less susceptible to stress and gain the most benefits from it.

L-lysine is a commonly recommended supplement for use in cats suffering from chronic herpesvirus infection. While evidence for use to treat or prevent upper respiratory symptoms is variable, it may demonstrate the most success in house cats who live in a cared-for and less stressful environment. Its lack of significant side effects and perception as a nutritional supplement makes it a favorable option to help with the chronic herpesvirus. 

Also Read: Gastroenteritis In Cats: Causes, Symptoms, & Treatment

Frequently Asked Questions

How much L-lysine should I give my cat?

The general recommendation for L-lysine supplementation for cats is up to 250mg for kittens or up to 500mg for adult cats, given once or twice a day. For most products available, this equates to about 1ml of gel, one chew, or a single scoop of powder. 

Can I give my cat L-lysine everyvday?

Yes. In fact, if you are using L-lysine as a supplement to help with herpesvirus symptoms, then it should be given every day. While L-lysine can be given during flare-ups as well, it likely has more preventive action for cats that show chronic signs of herpesvirus, if used consistently.

Can you give cats L-lysine?

Yes, but make sure to use a product intended or labeled for use in cats. Human supplements also exist, and should be avoided for use in cats. The forms intended for cats are less stressful to administer (in chew treat, tasty gel form, or as a flavored powder that can be mixed with food). Stress associated with the administration of L-lysine in cats may negate its potential benefits. 

Does Lysine for cats have side effects?

While digestive upset is possible with very high doses, significant side effects have not been routinely observed in cats receiving L-lysine at recommended, labeled doses.

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About Dr. Chris Vanderhoof, DVM, MPH

Dr. Chris Vanderhoof is a 2013 graduate of the Virginia-Maryland College of Veterinary Medicine (VMCVM) at Virginia Tech, where he also earned a Masters in Public Health. He completed a rotating internship with Red Bank Veterinary Hospital in New Jersey and now works as a general practitioner in the Washington D.C. area. Dr. Vanderhoof is also a copywriter specializing in the animal health field and founder of Paramount Animal Health Writing Solutions, which can be found at www.animalhealthcopywriter.com. Dr. Vanderhoof lives in the Northern Virginia area with his family, including 3 cats.

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