In an open-label study, 20 participants with major depressive disorder were given 250 mg of l-theanine daily in addition to their current medication. Treatment lasted 8 weeks, at the end of which Hamilton Depression Rating scores decreased significantly. Other benefits included significantly decreased anxiety scores, and improved sleep scores and measurements of cognition after treatment with l-theanine.
Hidese, S., Ota, M., Wakabayashi, C., Noda, T., Ozawa, H., Okubo, T., and Kunugi, H. (2017). Effects of chronic l-theanine administration in patients with major depressive disorder: an open-label study. Acta Neuropsychiatr. 29, 72–79. https://pubmed.ncbi.nlm.nih.gov/27396868/