Lysine is an amino acid that has been studied for its role in the management of cold sores (herpes labialis), which are typically caused by the herpes simplex virus type 1 (HSV-1). The rationale for its use is based on the fact that lysine may inhibit the replication of HSV-1 by antagonizing arginine, another amino acid that the virus requires for replication. Several clinical studies and reviews have explored the efficacy of lysine supplementation (usually in the range of 1,000–3,000 mg per day) for both prevention and treatment of cold sores.

Evidence from randomized controlled trials is mixed. Some studies have found that lysine supplementation can reduce the frequency, severity, and healing time of outbreaks, while others have found no significant benefit compared to placebo. For example, a meta-analysis published in 2017 concluded that lysine may be effective in reducing recurrence rates, but that the evidence is not uniformly strong across all studies. The variability in study design, lysine dosage, and patient populations makes it difficult to draw firm conclusions.

Overall, lysine is widely used for cold sores and there is some scientific basis for its use, but the strength of the evidence is moderate rather than conclusive. It is generally considered safe when taken in recommended doses, with few reported side effects.