The prescription of nutraceuticals is often based on theory and pre-clinical evidence (in-vivo or in-vitro) that a substance might behave in a particular way. However, we don’t know if something works on living humans with a specific condition until we conduct clinical trials and observational studies. The following result from a recent clinical trial is an example of the clinical efficacy not aligning with the pre-clinical evidence.
It’s still too early to draw definitive conclusions on the effect of L-theanine on anxiety symptoms, more research needs to be done to verify the results of this study.
You can read the study L-Theanine in the Adjunctive Treatment of Generalised Anxiety Disorder: A Double-Blind, Randomised, Placebo-Controlled Trial in the Journal of Psychiatric Research.