It is an amino acid that helps increase the immune system.  In keeping with my vet, Lysine is secure for cats and has no harmful unintended effects. Ideally search for a Lysine supplement in 625mg tablets in order that you do not have to worry about halving tablets or taking too much on a daily basis.

The cream docosanol (Abreva) is sold over the counter. In order to replicate, the herpes virus requires arginine , one other amino acid that is frequent in meals and necessary to human life