Does L-lysine work for cats?

The theory, and why it sounded good

Feline herpesvirus type 1 (FHV-1) is a leading cause of cat colds, runny eyes, and chronic upper-respiratory flare-ups, especially in shelters and multi-cat homes. The idea behind lysine is that the amino acid competes with arginine, which the virus needs to replicate, so loading a cat with lysine might starve the virus. In a test tube, that competition is real. The supplement industry built a large, durable product category on it.

The trouble is that a mechanism in a test tube is a hypothesis, not a result. The only way to know whether lysine helps a living cat is to give it to cats and count outcomes. That has been done, more than once, in the kind of studies that are hard to argue with.

What the controlled studies found

In 2015, Bol and Bunnik published a systematic review of lysine supplementation in cats in BMC Veterinary Research. Pooling the controlled evidence, they concluded that oral lysine does not prevent FHV-1 infection and does not reduce the signs of an active infection. They went further: they noted that some studies reported higherinfection rates and more severe disease in the lysine groups, and that lysine supplementation may even predispose cats to clinical signs (Bol & Bunnik 2015, PMID 26573523).

The two largest randomized trials underneath that review are the reason it lands so hard. Drazenovich and colleagues ran a randomized, placebo-controlled trial of dietary lysine in a shelter and found no benefit on eye or respiratory disease (Drazenovich 2009, PMID 19878022). Rees and Lubinski ran a randomized, placebo- controlled trial of oral lysine for preventing upper-respiratory disease in a shelter and, if anything, found the lysine group did slightly worse (Rees & Lubinski 2008, PMID 18547855). Both were exactly the high-stress, high-exposure settings where lysine is most often recommended.

Why a good product still scores Limited

This is the case our supplements rubric is built for. PetScored scores Evidence Quality as its single heaviest dimension, and it pins every active to the primary literature rather than the label. When we scored a lysine product, the Vetoquinol Enisyl-F oral paste, almost everything about the execution was good: it is NASC-sealed, it is dosed at the amount the studies used, and the label is restrained rather than overreaching. None of that could rescue the Evidence Quality score, because the controlled evidence says the active does not deliver the outcome it is sold for. The product lands in our lowest band as a result.

That is the point of separating evidence from execution. A skeptic site that scored manufacturing quality alone would rate a well-made lysine paste highly and mislead the buyer. The honest read is that you can do everything right with an ingredient that does not work, and the rubric has to say so.

What this does and does not mean

It means the weight of controlled evidence is against oral lysine for FHV-1. It does not mean any individual cat that seemed to improve on lysine was imagined; colds wax and wane on their own, which is exactly why anecdotes are unreliable and randomized trials exist. And it does not mean a cat with chronic herpesvirus has no options: environmental stress reduction, managing co-infections, and antivirals your veterinarian may prescribe are real levers. Lysine just is not the lever the label implies.

- The PetScored desk. See how we tier evidence in the supplements methodology, and the scored lysine product on the Enisyl-F scorecard.