Humans occasionally, very occasionally, are useful. Celia, whose studies I find extremely irritating, nevertheless was able to research several papers about Lysine. Apparently about 90% of animal hospitals in the USA, UK and Australia give Lysine to cats that have the Feline Herpes Virus 1 (FHV1). FHV1 causes "goopy eye" in cats, stays in the body life long and like cold sores in humans is likely to surface during times of stress.
The theory of giving Lysine, a food supplement, was that it reduced arginine levels, an amino acid, which in turn reduced the herpes virus's ability to reproduce. Thus cats treated with Lysine shed less virus and would recover faster. It's a great idea. And because an attack of FHV (like cold sores) usually goes away eventually anyway, vets thought it was the Lysine that was working.
But it doesn't work. In practice it makes no difference at all. The latest scientific paper on this drug says it is ineffective and should not be used (details in a PS). This isn't just because it doesn't work, it is because cats need arginine, an essential amino acid. So lowering the level of arginine may be bad for the cat's health. I suggest Bentley gets his human to find the full paper on Google Scholar, downloads it, and shows it to his vet.
It looks as if it would be better for Bentley's humans to concentrate on giving him a stress free life (look here). He needs good nutrition, lots of interesting things to do (look here) if he is an indoor-only cat, a settled routine, and sensitivity to how much affection he wants from humans.
Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review
Background: Feline herpesvirus 1 is a highly contagious virus that affects many cats. Virus infection presents with flu-like signs and irritation of ocular and nasal regions. While cats can recover from active infections without medical treatment, examination by a veterinarian is recommended. Lysine supplementation appears to be a popular intervention (recommended by > 90 % of veterinarians in cat hospitals). We investigated the scientific merit of lysine supplementation by systematically reviewing all relevant literature.
Methods: NCBI’s PubMed database was used to search for published work on lysine and feline herpesvirus 1, as well as lysine and human herpesvirus 1. Seven studies on lysine and feline herpesvirus 1 (two in vitro studies and
5 studies with cats), and 10 publications on lysine and human herpesvirus 1 (three in vitro studies and 7 clinical trials) were included for qualitative analysis.
Results: There is evidence at multiple levels that lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats. Lysine does not have any antiviral properties, but is believed
to act by lowering arginine levels. However, lysine does not antagonize arginine in cats, and evidence that low intracellular arginine concentrations would inhibit viral replication is lacking. Furthermore, lowering arginine levels is highly undesirable since cats cannot synthesize this amino acid themselves. Arginine deficiency will result in hyperammonemia, which may be fatal. In vitro studies with feline herpesvirus 1 showed that lysine has no effect on the replication kinetics of the virus. Finally, and most importantly, several clinical studies with cats have shown that lysine is not effective for the prevention or the treatment of feline herpesvirus 1 infection, and some even reported increased infection frequency and disease severity in cats receiving lysine supplementation.
Conclusion: We recommend an immediate stop of lysine supplementation because of the complete lack of any scientific evidence for its efficacy.