Lysine is an essential amino acid needed for growth and to help maintain nitrogen balance in the body. (Essential amino acids cannot be made in the body and must be supplied by the diet or supplements.)

Lysine appears to help the body absorb and conserve calcium. Lysine has many functions in the body because it is incorporated into many proteins, which are used by the body for a variety of purposes. Lysine interferes with replication of herpes viruses and is therefore often taken as a supplement to help to alleviate symptoms of cold sores. A review of the research trials investigating the effects of lysine on people with cold sores shows that most, though not all, trials support the use of lysine.