Forms of l-lysine

My Wellington has been on l-lysine now for over a year, since he was diagnosed with chronic feline herpes. Prior to that he was tested for allergies, parasites and behavioural problems causing him to pull all the fur out on his belly and hind legs. The poor boy was really suffering. Anyway, I started giving him l-lysine daily and the herpes got under control, though he always had weepy eyes, however much I upped the dosage (and I did not want to overdo it, though he is a very big cat). The only way I could buy it here was in tablets, human prescription, and no vet here really understood what I was doing, though did not disagree with the treatment or dosage.

So Wellington has been taking tablets twice daily for over a year; he is very good about it, though I know he hates it and hides from me if I have something hidden in my hand. He did not stop the overgrooming and we could not solve that problem.

Then on TCS I heard about someone who was prescribed Enisyl by the vet, and learnt that it was a cat form of l-lysine. No one knew about it here, but I discovered I could buy it online. Two tubes arrived a couple of weeks ago, and I have been giving the prescribed dose to Wellington. What a difference! He loves the fishy flavour - it is so much easier to squirt the paste into his mouth, and his eye is clear for the first time in months. The amazing thing is that he has stopped pulling fur out too, and his stomach is already covered with a fine down. Maybe he was doing it just because he felt so miserable.

I almost never recommend products, but this stuff has transformed my boy and has made my relationship with him so much better. I felt so bad that he wouldn't come near me - now he is cuddling me again. He does have sardine breath though, but I can put up with that!

My new kitties also were diagnosed with suspected herpes virus . The vet told me they've had excellent results with lysine and gave me a tube of Viralys oral gel (lysine) which my kitties also took to.  They lick it off my finger. 

One caution about L-lysine sold for humans, even the kind you get at the health food store.  Most lysine sold for humans, even the '100% natural' ones contain Polyethylene Glycol which is toxic to cats.  If you are using Lysine that isn't formulated for kitties, be sure to check the ingredients.

TobyTyler said:

One caution about L-lysine sold for humans, even the kind you get at the health food store.  Most lysine sold for humans, even the '100% natural' ones contain Polyethylene Glycol which is toxic to cats.  If you are using Lysine that isn't formulated for kitties, be sure to check the ingredients.

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You are absolutely right and I should have mentioned it. But it can be very hard to get anything that does not have a 'binding' agent in it.

jennyranson said:

 But it can be very hard to get anything that does not have a 'binding' agent in it.

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What I don't get is how Polyethylene Glycol could be considered 100% natural!  I take L-Lysine occasionally myself for cold sores (Herpes Simplex Virus).

So when I got the Viralys gel from the vet, I looked at my own lysine to see if maybe I could just give that to them.  I got the lysine from a health food store, thinking it was just L-Lysine since it said '100% natural'.   Sure enough, one of the ingredients is P-Glycol

I guess we need to look at all ingredients on everything we and our kitties eat, regardless of where purchased.

I don't know if this can be considered hijacking your thread (I hope not), but I was reading the thread because I suspected my cats have eye herpes, and that I've been giving them OTC l-lysine for humans. I'm glad to know about the cat-friendly l-lysine but now I'm alarmed because your posts and the other posts say that polyethylene glycol is toxic to cats and my cat was prescribed medication for intestinal problems that includes 1/4tsp of polyethylene glycol twice a day. What is it about polyethylene glycol that makes it toxic for cats? Is it any amount of it so all human products that use it as a binding agent coould be problematic?

I don't know about Europe but in the USA you can get pure l-lysine powder. It is nothing but l-lysine. No other additives. You put the powder in their food, the only taste is a slightly salty taste. Cats love salt, so for most cats, this is not a problem.

I use Source Naturals brand, simply because I have always used it. Other members here prefer the NOW brand.

Actually, I think we are all a bit confused on this one, though it is not surprising. I just did a google search and this is some of what I came up with:

Herpesviruses as a group are highly dependent on the presence of an amino acid called arginine. Without arginine, Herpes cannot reproduce. The amino acid lysine is taken up by the virus in favor of arginine. We can take advantage of this situation by saturating the virus with lysine and thus suppressing the virus' ability to replicate. Lysine is readily available in most health food stores as a tablet or capsule. One should be sure that the formula used is free of the preservative propylene glycol as cats can have blood reactions against this compound. A month or so of supplementation is required in order to determine if supplementation has been helpful


Propylene glycol is an approved food additive for dog food under the category of animal feed and is generally recognized as safe (GRAS[30]) for dogs.[31] Similarly, propylene glycol is an approved food additive for human food as well.[32] The exception is that it is prohibited for use in food for cats because of a species-specific reaction in the body, as noted in 21 CFR 582.1666.

Veterinary data indicate that propylene glycol is toxic to dogs with a 50% chance of being lethal at doses of 9mL/kg, although the figure is higher for most laboratory animals (LD50 at levels of 20mL/kg).[33]

However, propylene glycol may be toxic to cats in ways not seen in other animals. The U.S. Food and Drug Administration has determined that its presence in or on cat food has not been shown by adequate scientific data to be safe for use. Any such use is considered an adulteration of the cat food and a violation of the Federal Food, Drug, and Cosmetic Act.[34]

propylene glycol is CH2OH-CHOH-CH2OH. It is poisonous. So is its kid brother ethylene glycol, which is the main ingredient in car anti-freeze. However, the polyethylene glycol is a POLYmer, a molecule that is formed from a base molecule (in this case, ethylene glycol), which is the "repeating unit" in the molecule. The molecule may be hundreds or thousands of units long. In this case, the -OH groups on neighboring molecules are "condensed", yielding one molecule of water, and an "O-O" bond between the two molecules. When this is repeated, you get a polymer. So you have
HO-CH2CH2-O-CH2CH2-O-many times-CH2CH2OH. This is NOT poisonous, and is a common ingredient of gel tablets.


So it seems that polyetheline glycol is OK, but propylene glycol is not. Unfortunately some list of ingredients simply say 'p-glycol' so how is one to know?

*sigh of relief * thanks. And as you pointed out, it's best to stay.awayfrom"p-glycol". Hyphenating that should be illegal since there's more than one glycol that starts with a p.

jennyranson said:

Actually, I think we are all a bit confused on this one, though it is not surprising. I just did a google search and this is some of what I came up with:
Herpesviruses as a group are highly dependent on the presence of an amino acid called arginine. Without arginine, Herpes cannot reproduce. The amino acid lysine is taken up by the virus in favor of arginine. We can take advantage of this situation by saturating the virus with lysine and thus suppressing the virus' ability to replicate. Lysine is readily available in most health food stores as a tablet or capsule. One should be sure that the formula used is free of the preservative propylene glycol as cats can have blood reactions against this compound. A month or so of supplementation is required in order to determine if supplementation has been helpful
Propylene glycol is an approved food additive for dog food under the category of animal feed and is generally recognized as safe (GRAS[30]) for dogs.[31] Similarly, propylene glycol is an approved food additive for human food as well.[32] The exception is that it is prohibited for use in food for cats because of a species-specific reaction in the body, as noted in 21 CFR 582.1666.
Veterinary data indicate that propylene glycol is toxic to dogs with a 50% chance of being lethal at doses of 9mL/kg, although the figure is higher for most laboratory animals (LD50 at levels of 20mL/kg).[33]
However, propylene glycol may be toxic to cats in ways not seen in other animals. The U.S. Food and Drug Administration has determined that its presence in or on cat food has not been shown by adequate scientific data to be safe for use. Any such use is considered an adulteration of the cat food and a violation of the Federal Food, Drug, and Cosmetic Act.[34]
propylene glycol is CH2OH-CHOH-CH2OH. It is poisonous. So is its kid brother ethylene glycol, which is the main ingredient in car anti-freeze. However, the polyethylene glycol is a POLYmer, a molecule that is formed from a base molecule (in this case, ethylene glycol), which is the "repeating unit" in the molecule. The molecule may be hundreds or thousands of units long. In this case, the -OH groups on neighboring molecules are "condensed", yielding one molecule of water, and an "O-O" bond between the two molecules. When this is repeated, you get a polymer. So you have
HO-CH2CH2-O-CH2CH2-O-many times-CH2CH2OH. This is NOT poisonous, and is a common ingredient of gel tablets.
So it seems that polyetheline glycol is OK, but propylene glycol is not. Unfortunately some list of ingredients simply say 'p-glycol' so how is on  e to know?

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That is good to know!  Thank you.