L-lysine is an amino acid that is used in humans and animals for controlling herpes viral infections. Lysine interferes with the virus uptake of arginine. This amino acid is required for replication. With inadequate amounts of arginine, the virus can’t reproduce decreasing the number of organisms in the patient. Many people use it to decrease the occurrence of cold sores. In veterinary medicine, it is primarily used for treating chronic herpes viral infections in cats. My own cat Tigre, is chronically infected with the herpes virus. The number and duration of outbreaks decreased dramatically when I placed him on a low dose of lysine. Some equine veterinarians prescribe lysine for a condition called ‘pastern dermatitis.’ Horses with this condition develop a vasculitis in the skin of their pasterns. The condition improves when the horses are treated with lysine or a mixture of lysine and flaxseed oil.
Unfortunately, lysine can cause problems if not used properly. Besides depleting arginine in a virus, it can theoretically do the same thing in animals. I recently read about a cat who was started on a moderate dose of lysine as a kitten for a herpes infection. At ten years of age, the cat started having episodes of strange behavior that included dizziness, vocalization, drooling and vomiting. During the episode, the cat walked around like it was drunk. The clinical signs started after the cat received its dose of lysine and lasted for about four hours. The episodes stopped when the lysine was discontinued. A clinical study by Dr. Fascetti and colleagues was not able to replicate these signs when challenging cats with excess dietary lysine. Until more is learned about lysine-arginine antagonism I recommend giving a low dose of lysine no more than twice a day instead of placing it in the food.
L-lysine comes in a variety of forms and from many different sources. Although the lysine is the same, other ingredients are often added to make it more palatable. Please be careful with these added ingredients. I have read reports of some companies using artificial sweeteners and/or propylene glycol to their products which are harmful to animals. If a label states that other ingredients are added to improve taste, find out what they are before giving them to your pet. If the company refuses to give all the ingredients, find a new product. If the ingredients are not classified as ‘Generally Recognized as Safe’ (GRAS) by the Food and Drug Administration I would avoid the product. More information including a list of GRAS ingredients can be found at http://www.fda.gov/Food/FoodIngredientsPackaging/GenerallyRecognizedasSafeGRAS/default.htm.
-Fascetti, A.J., et al., Excess dietary lysine does not cause lysine-arginine antagonism in adult cats. J. Nutr. August 2004; 134(8 Suppl): 2042S-2045S.
-Warren, E. ‘Nutraceuticals’ The VSPN notebook, 4/4/2007.