L-theanine is a water-soluble amino acid. Amino acids are the building blocks of proteins. L-theanine has been studied for its possible health benefits, ranging from cancer and stroke prevention to weight loss. However, most research on the amino acid has been conducted to assess its stress-relievingeffects.1

L-theanine is found mainly in tea (eg, green tea and black tea), and to a less extent in mushrooms. L-theanine is also available in its purified form as an oral dietary supplement.1