Does leucine work for cold sores?

No: This will not help with cold sores. You can try abreeva.

It's Lysine: I do believe the Amino Acid you are referring to is lysine. Like everything else in the way of meds...It may work for others and may not work for you. But, you have nothing to lose....So head off to buy a bottle and give it a try. Take it daily and take it regularly. Yu have to give it a try and don't stop after a few days.