Sisu L-LysineReduces the recurrence and severity of cold sores Accelerates healing process L Lysine is an essential amino acid, meaning the body needs it but cannot manufacture it so you must consume enough in your diet or in a supplement. The best food sources are red meat, fish, dairy, eggs, beans, and legumes. By competing with L Arginine, another amino acid, L Lysine helps counteract the Herpes virus that causes cold sores when the immune system is weakened.