Â Lysine Blocks Cold Sores, A Reason to Smile Between 60-90% of adults carry the virus that causes cold sores. These eruptions around the mouth and on the lips, often triggered by excess sunlight or stress, are both unsightly and painful, and can last for up to a week. In the 1980s, scientists discovered that the amino acid L-Lysine could help block the recurrence of cold sores by reducing the virusâ€˜ ability to replicate. Cold sores require arginine to replicate and L-Lysine inhibits the absorption of arginine.Â Â Though L-Lysine is abundant in animal proteins, , itâ€˜s believed that supplementing with L-Lysine at the onset of a cold sore can hasten healing and prevent recurrences. Natureâ€˜s HarmonyÂ® L-Lysine is a trusted source for this essential amino acid.