Dosage I have seen recommended is often 500mg per day, divided over two doses (so 225mb twice a day). But I have also seen it suggested up to 1000mg per day for intensive therapy. It takes about a week for symptoms to resolve with Lysine treatment.

I personally have never seen improvement with Lysine. It was recommended for Ringo by all the vets when we first adopted him at 6 mos. and it seemed to do absolutely nothing. I’ve had multiple vets suggest this for him. Interestingly, studies in the last couple decades seem to show what I experienced – it really doesnt help and I’ve just read that a lot of vets are now backing away from this, though our vet recently recommended this again not too long ago. Currently I’m giving it to Pinky since yesterday, but after reading these studies which echo what I found in the past, I might stop in favor of trying something else for her that might have a better chance of helping her. This is especially because it has some interaction with Arginine which was the basis of its supposed effectiveness as explained below – however, Arginine detoxifies ammonia in cats’ kidneys, is important for heart and pancreatic function, and helps keep the gut healthy. So, if its not actually effective in helping her upper respitory issues, then I certainly don’t want to potentia;y diminish the postive effectives of arginine for her (not sure that it would, but just in case).

Sneezing and sniffling: those are sounds that every cat parent dreads. Cat respiratory infections —kitty colds—are common illnesses that are difficult for a cat to shake. Often, veterinarian appointments and prescribed medications are necessary to help a cat recover from her cold.

Along with conventional treatments, many veterinarians prescribe L-lysine supplementation to cats suffering from respiratory problems or illnesses like the feline herpesvirus.

Lysine is an essential amino acid that helps produce antibodies and aids in calcium absorption. This is important, so our kitties can fight diseases and have healthy bones. Cats do not make lysine on their own. They must obtain it through their food and supplements. 

In cats, Lysine is used to fight the feline herpes virus (FHV-1) in cats.

FHV-1 is comparable to the common cold in humans. It shows up through nasal discharge, conjunctivitis, and other upper respiratory ailments. FHV-1 is extremely common, and it is likely that most cats have been exposed to it at some point. Some just have better immune systems to fight it and rarely show signs. Others can get severe cases and have secondary infections occur. Below is a list of typical clinical signs of FHV-1:

  • Coughing
  • Sneezing
  • Runny nose/ productive discharge
  • Conjunctivitis
  • Lethargy
  • Inappetance
  • Squinting of eyes
  • Wheezing

In order for FHV-1 to replicate, it needs to bind with the amino acid arginine. Lysine works by interrupting that relationship, resulting in an improvement in the clinical signs.

Arginine, an amino acid found naturally in the body, is essential to some viruses when replicating (i.e. creating more virus) including herpesviruses. L-lysine is supposed to stimulate an enzyme in the liver that breaks down arginine. 

The theory is that when high levels of L-lysine are consumed, the body secretes the enzyme that breaks down arginine. With less arginine, feline herpesvirus should not be able to make as much virus and your cat should feel better. The full effect of L-lysine supplementation takes a few weeks before pet parents can expect results.

FHV-1 is likely to flare up when cats’ immune systems are weakened through concurrent diseases or stress. It is also common for cats to develop secondary infections because their immune system is not in tip-top shape and has trouble fighting any foreign pathogens. In these cases, cats can be given anti-virals, antibiotics, along with lysine to help fight the diseases taking over. 

Cats with feline herpes will display a respiratory infection with flu-like symptoms like wheezing, fever, lethargy and conjunctivitis (swollen eyes with discharge). Many cats experience exposure to the feline herpes virus however never show symptoms of the illness. If your cat has been exposed to the virus and his immune system becomes weak, the virus may flare up and cause symptoms in your cat. Lysine may also be used for cats that are experiencing allergies.

Most cats with upper respiratory symptoms have feline herpesvirus. This particular virus causes symptoms off and on for life, waxing and waning during times of stress such as moving, vacations, or after other illnesses or procedures such as surgery. 

The goal of L-lysine is to decrease symptoms and prevent flare-ups over time, not as a short-term treatment.

Side effects of lysine are rarely seen; however, no medication or supplement is without them. By using lysine, you are blocking the amino acid arginine. Without this amino acid, cellular and organ function can be disrupted. 

A lot of research has been done involving L-lysine and has mostly shown L-lysine to have no effect on the health and wellbeing of cats. Veterinarians are now more reluctant to recommend this supplement, although generally L-lysine is not harmful to most cats. 

Multiple studies have demonstrated that when given in food, lysine was found to be ineffective in preventing disease [1] [2].In one such study, the cats receiving lysine experienced worse symptoms and higher levels of feline herpesvirus when tested [1].

A review of multiple studies performed both in cats and in humans concluded that lysine is not effective against herpesviruses in general [3].The theory that lysine decreases arginine is shown to be false in several studies. There is no evidence that low levels of arginine in the body stop herpesvirus from producing more virus or that increased L-lysine supplementation decreases herpesvirus. After decades of research, it would appear lysine is not an effective treatment for feline herpesvirus after all.

It has also been noted that lysine can cause allergic reactions and can stay in the system longer for any cats with kidney or liver disease. It has also been reported that it may cause diarrhea. 

ide effects are uncommon with L-lysine. Cats that eat too much L-lysine may experience gastrointestinal side effects including:

If your cat starts taking an L-lysine supplement and begins experiencing these symptoms, call your veterinarian and double check the recommended dosing.

ide effects are uncommon with L-lysine. Cats that eat too much L-lysine may experience gastrointestinal side effects including:

If your cat starts taking an L-lysine supplement and begins experiencing these symptoms, call your veterinarian and double check the recommended dosing.

Dosing of L-lysine for cats is generally 250 mg to 500 mg once or twice per day. Kittens and smaller adult cats should tend towards the lower dose, while larger adult cats are usually fine receiving the higher dose. There is no specific dose based on weight. 

Speak with your veterinarian for dosing guidance, and always follow the instructions on the supplement’s container unless your veterinarian advises otherwise.

Unfortunately, while lysine has few side effects, some cats will experience an allergy to it.

An amino acid given as a supplement, lysine can be helpful in keeping the herpes virus under control. While it has minimal side effects, should your cat’s immune system overreact, he will experience an allergy.

Cats with feline herpes will display a respiratory infection with flu-like symptoms like wheezing, fever, lethargy and conjunctivitis (swollen eyes with discharge). Many cats experience exposure to the feline herpes virus however never show symptoms of the illness. If your cat has been exposed to the virus and his immune system becomes weak, the virus may flare up and cause symptoms in your cat. Lysine may also be used for cats that are experiencing allergies.

Adjunct Treatments With Lysine

According to one lysine distributor, lysine for cats often accompanies other treatments. To ease the discomfort of the conjunctivitis and to aid in healing, veterinarians often recommend an eye ointment containing an anti-bacterial agent, such as tetracycline. Prescription drugs might be administered by injection to ease inflammation or reduce the recurrence of outbreaks.

To help relieve the strain of the respiratory congestion, herbs like olive leaf, propolis and grapefruit seeds have long been used to relieve congestion in humans and their pets alike. Other supplements might be effective in controlling feline respiratory infections, from lactoferrin to colostrum.

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Symptoms of Lysine Allergy in Cats

  • Itchiness in your cat’s skin
  • Excessive licking
  • Biting his skin 
  • Skin infection is possible
  • Fur loss may be seen
  • His skin may appear inflamed


 Cats can experience different types of allergies:

Contact Allergies

These occur when your cat’s skin comes in contact with a particular allergen. The allergen will cause a reaction in his skin. Items in your home like disinfectants or chemicals can lead to contact allergies in your cat. 

Flea Allergy Dermatitis

This happens when the saliva from a flea touches your cat’s skin and leads to an allergy. This will lead to itchy skin; your cat’s biting and chewing the areas that are impacted can cause lesions in his skin and his hair can come out in patches.

Inhalant Allergy

This can develop when your cat breathes in particular allergens that are in the air; for example: mold, dander, spores and cigarette smoke. In the case of an inhalant allergy, you may see watery eyes, nasal discharge, coughing and breathing trouble.

Food Allergy

Cats can develop allergies to foods they have consumed for a long period of time. Common allergens include chicken and dairy products.

Causes of Lysine Allergy in Cats

An allergy in your cat will occur when his immune system over-reacts to a protein in something that he is consuming. The response of his immune system is what will cause him to experience itching, inflammation and other symptoms.

Diagnosis of Lysine Allergy in Cats

Should you notice concerning symptoms in your cat, you will want to visit your veterinarian, who will conduct a physical examination. Your veterinarian will ask you questions in regards to the symptoms you have noticed, when you first noticed them and any changes that have occurred. You will also be asked for details in regards to his diet, medications and supplements that he takes and whether there have been any recent changes.

Depending on the physical examination, your veterinarian may conduct a complete blood count, serum chemistry analysis, urinalysis and stool sample evaluation. These will help your veterinarian determine if there is an underlying illness that is leading to your cat’s symptoms. Your veterinarian may obtain a sample of your cat’s skin cells and view them under a microscope to see if mites, fungus or bacteria may be causing your cat’s symptoms. 

If your veterinarian feels that your cat is experiencing an allergy to something he is consuming, like lysine, he will work with you on developing a plan to eliminate different elements of his diet. Should your cat be taking different supplements, your veterinarian may recommend eliminating them first to see if his symptoms are resolved. You can then add them back in one at a time and see if the allergy is triggered, thereby confirming what your cat is reacting to.

Treatment of Lysine Allergy in Cats

Should it be determined that your cat is experiencing an allergy to lysine, it is important to eliminate the supplement from your cat’s diet. If your cat has been exposed to the herpes virus and lysine has been helping to strengthen his immune system, your veterinarian may be able to recommend other supplements that can replace lysine.

Recovery of Lysine Allergy in Cats

Once lysine is removed from your cat’s diet, the symptoms he is experiencing should resolve themselves. It is a good idea to work closely with your veterinarian to find other options that will help your cat strengthen his immune system. A strong immune system is particularly important if he has been exposed to the herpes virus as his immune system can keep the virus in check so that symptoms are not experienced.

Cats are highly susceptible to upper respiratory infections, which may share symptoms with FHV-1. The following symptoms may indicate general respiratory illnesses or the feline herpesvirus:

  • Nasal congestion
  • Sneezing
  • Fever
  • Sore throat
  • Discharge from eyes, mouth, and nose
  • Lack of appetite
  • Lethargy
  • Conjunctivitis
  • Panting

Upper respiratory illnesses may be caused by either a bacterial or viral infection, and chronic occurrences of these colds may signal the existence of FHV-1. Cats with weaker immune systems, such as kittens or senior cats, are most prone to respiratory infections and FHV-1; however, even healthy cats in their prime can fall victim to these illnesses.

Consult your veterinarian to determine if your cat is dealing with a kitty cold or a persistent bout of FHV-1. In particular, FHV-1 can be tricky, as most infected cats will overcome the initial infection, but some will develop a chronic condition. In these cats, the immune system is suppressed and FHV-1 will flare-up and cause the symptoms to start all over again.

Veterinarians typically prescribe L-lysine supplements because of a long-held assumption that the amino acid eliminates the aforementioned symptoms of the infections and also prevents future flare-ups from occurring; nearly 90% of veterinarians recommend the use of L-lysine to treat these conditions. Veterinarians were of the thought that L-lysine interfered with the replication of the FHV-1 virus by preventing the uptake of a second amino acid, arginine.

In the early 2000s, a series of research was conducted on the use of L-lysine on cats with varying and, at times, conflicting and inconclusive results. A 2003 study in the American Journal of Veterinary Research found a reduction in FHV-1 virus replication with an oral dose of 400mg of L-lysine.

However, a 2009 study, also published in the American Journal of Veterinary Research, argued that L-lysine was not an effective management tool for countering the FHV-1 virus, and might make the infection worse. The study examined a group of 261 cats and found that dietary lysine supplementation had no influence on controlling or preventing infectious upper respiratory disease amongst these felines.

Despite the lack of definitive proof of L-lysine’s effectiveness, many veterinarians continue to prescribe it for cats with respiratory and feline herpesvirus, as no better alternative (other than supportive care) exists to address respiratory illnesses at this time.

A New Study Sheds Light on L-lysine For Cats

The veterinary community’s attitude toward L-lysine and cats began to change when a 2015 study was published in the BMC Veterinary Research journal. The researchers found that there was no proof that L-lysine supplementation suppressed the feline herpesvirus in infected cats. They recommended that veterinarians stop prescribing L-lysine immediately due to the lack of evidence concerning its effectiveness. The study was summed up as follows:

  • No evidence was found that L-lysine prevented cats from FHV-1 infection
  • L-lysine did not prevent recurrences of FHV-1 and its symptoms
  • No proof exists that L-lysine can alleviate FHV-1 and its symptoms

Frequent use of L-lysine as a supplement lowers levels of arginine, an amino acid essential to cats; low arginine levels may cause serious health issues in felines. Arginine is responsible for critical body functions, such as kidney filtration, immune system function, hormone function, and wound healing.

Lysine and arginine compete with each other for absorption within the body. Initially, researchers thought that lower arginine levels helped prevent FHV-1—hence the L-lysine recommendation—but the 2015 study has caused scientists, researchers, and veterinarians to adopt a more cautious attitude toward the use of L-lysine.

Worse yet, a cat with arginine deficiency may die from hyperammonemia (ammonia intoxication), as cats are unable to synthesize arginine. Because of these conclusions, some veterinarians have reversed their position on L-lysine and no longer recommend its use on cats although it was never shown that such a severe symptom resulted from lysine administration.

Other Side Effects of L-lysine on Cats

L-lysine can negatively impact cats in other, less damaging ways, which can nonetheless be uncomfortable for your cat. These side effects include:

  • Seizures
  • Nausea
  • Vomiting
  • Persistent scratching
  • Diarrhea
  • Facial swelling
  • Pale gums

L-lysine supplementation may also interfere with other medications that your cat is taking. Always talk to your veterinarian about other medications, supplements, or vitamins that your cat is taking before beginning L-lysine treatment. Large doses of L-lysine may interact with certain antibiotics and increase the antibiotics’ toxicity.

Cats with digestive disorders, such as irritable bowel syndrome or ulcers, might find their chronic conditions worsened when taking L-lysine supplementation, due to negative interactions with other prescribed medications. Occasionally, a cat may develop an allergy to the L-lysine medication, resulting in difficulty breathing, dizziness, and rashes.

Leave the L-lysine Behind

Because of the uncertainty surrounding its effectiveness, and the findings of the 2015 study, some veterinarians are more reluctant to prescribe L-lysine. Other doctors are waiting on new studies to be completed and results published. Until more information is available, err on the side of caution and discuss with your veterinarian whether L-lysine is right for your cat.

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