L-Theanine-d 5 is the deuterium labeled L-Theanine. L-Theanine (L-Glutamic Acid γ-ethyl amide)is a non-protein amino acid contained in green tea leaves, which blocks the binding of L-glutamic acid to glutamate receptors in the brain, and with neuroprotective and anti-oxidative activities. L-Theanine causes anti-stress effects via the inhibition of cortical neuron excitation by oral intake.