This 2019 randomized double blind, placebo controlled study followed 44 subjects with Generalised Anxiety disorder (GAD) resistant to their current antidepressant medications. While this study did not demonstrate statistical significant in primary outcomes (sleep, cognition, and anxiety rating) over placebo there was an improvement in subjects self-reported sleep satisfaction. Further the researchers suggest that there may be a more pronounced benefit in those with mild and severe insomnia. Regarding management of GAD, researchers support the hypothesis that L-theanine may be more indicated for individuals with subclinical or secondary anxiety.
Sarris J, Byrne GJ, Cribb L, Oliver G, Murphy J, Macdonald P, Nazareth S, Karamacoska D, Galea S, Short A, Ee C, Birling Y, Menon R, Ng CH. L-theanine in the adjunctive treatment of generalized anxiety disorder: A double-blind, randomised, placebo-controlled trial. J Psychiatr Res. 2019 Mar;110:31-37. doi: 10.1016/j.jpsychires.2018.12.014. Epub 2018 Dec 8. PMID: 30580081.