L-Lysine is recommended by 'some' vets, but should be used with caution!!! L-lysine tabs contain sulphites which can cause breathing difficulties in some cats... There is not tablets made specifically for cats, you would have to go to say Holland and Barrett and buy tablets there and cut them into quarters which is tricky... L-lysine 'can' be good for the immune system in cat flu/FIV+ cats. but would always consult a vet before doing so.... ie. its use in asthmatic cats could be fatal due to tablets containing sulphites.

ps.just wanted to add 'food suppliements' are not always 'safe' to use. You should ALWAYS consult your vet first! Food suppliements can effect liver/kidney function.