Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials

Researchers combined results from 31 randomized trials, where people were assigned by chance to take L-theanine or a placebo, an inactive comparison. The studies tested effects on attention, thinking, mood, and stress. Results varied across studies, with some signs of small benefits and other tests showing no clear difference. The trials used different doses and measured effects over different time periods.

What this means for you: L-theanine may provide a modest short-term benefit for some people, but this review does not show a reliable, broad improvement in focus or mood. It is not strong enough evidence to justify buying it specifically for cognitive or emotional benefits.