Lysine is an essential amino acid in human nutrition because the body cannot produce it; therefore, it must be taken in either by diet or supplementation. Lysine was first isolated from casein (a milk phosphoprotein) in 1889. Lysine has been studied for the prevention and treatment of herpes infections and cold sores. It also increases the intestinal absorption of calcium and eliminates its excretion by the kidney, suggesting that it might be helpful in osteoporosis. Patients who are pregnant and in lactation phase or with liver and kidney impairment should avoid supplementation with lysine.