In the 1980s, before there were effective drugs for cold sores (caused by a herpes virus, which can also cause genital outbreaks), supplements of the amino acid lysine were regarded as a potential treatment and preventive. The idea that lysine, an amino acid, might work against herpes has some plausibility. The herpes virus requires arginine, another amino acid, in order to replicate; lysine is thought to interfere with the absorption of arginine in the intestine. Lysine is supplied by many foods—notably red meats, fish, and dairy products. You can certainly satisfy your needs for lysine from dietary sources. But to fight herpes, you would need more, the thinking ran.
What the science says: It’s a long way from this observation to a herpes treatment, and most studies on high doses of lysine as a preventive for cold sores have been inconclusive. While a few studies have found that lysine decreases the severity or duration of an outbreak, others have not. In any case, most of the studies have been small and poorly designed.
Common side effects: Serious side effects (notably kidney problems) from long-term use have been reported.
Our advice: We do not recommend lysine. Its effect is unproved. Instead, there are safe and effective medications for the treatment and prevention of herpes outbreaks. A caution: If you do take lysine supplements, take them only when you feel a cold sore coming on, not continually.