Theanine is a unique amino acid present nearly exclusively in three species of tea plants. It is also present in one species of mushrooms. It is not present in proteins like other amino acids are. One to two percent of the weight of green tea leaves is L-theanine.

The green tea plant has become a popular food and dietary supplement recently because it seems to promote certain aspects of good health including relaxation and healthy serotonin levels. It is the L-theanine in the green tea that is associated with these health benefits.

For those individuals who do not drink teas, L-theanine can still be consumed by using a dietary supplement of L-theanine.