Lysine, or L-lysine, is an essential amino acid, which means it is necessary for human health. However, our bodies cannot manufacture it; lysine has to be absorbed from the food we eat. Amino acids like lysine are the building blocks of protein. Lysine is particularly important for proper growth, and plays an essential role in the production of carnitine, a nutrient responsible for converting fatty acids into energy and helping to lower cholesterol. Lysine also appears to help the body absorb calcium, and play an important role in the formation of collagen, an important component of bones and connective tissues including skin, tendon, and cartilage.

Some studies have found that taking lysine on a regular basis may help prevent outbreaks of cold sores and genital herpes. Not every study has shown positive results, however. One study found that taking lysine at the beginning of a herpes outbreak did not reduce symptoms.

Lysine helps the body absorb calcium and decreases the amount of calcium that is lost in urine. Because calcium is crucial for bone health, some researchers think lysine may help prevent bone loss associated with osteoporosis. Lab studies suggest that lysine in combination with L-arginine (another amino acid) makes bone-building cells more active and enhances production of collagen. But no studies have examined whether lysine helps prevent osteoporosis in humans.