Tea and Teapot, Youqing Wang
Green tea’s calming amino acid has good evidence for attention, weaker support for mood and stress
STUDY: Gerolymos C et al, Molecular Psychiatry 2026
STUDY TYPE: Systematic review and meta-analysis
FUNDING: Independent
Background
L-theanine is an amino acid in green tea that crosses into the brain and raises alpha-wave activity, GABA, and dopamine signaling. It’s sold as a calming, focus-boosting supplement, and this review looks at the evidence behind those claims.
The Study
- 31 randomized trials, 1,168 participants, mostly healthy adults.
- Compared oral L-theanine, usually a single 200 mg dose, with placebo, or 200 mg bid.
- Outcomes included stress, anxiety, depressive symptoms, fatigue, and reaction time.
Results
A single dose sped up choice reaction time (effect size 0.51), the most solid finding in the review. The benefit showed up within about two hours of dosing, in line with l-theanine’s 72-minute half-life. Studies that tested cognition after that window usually turned up negative.
l-Theanine reduced stress symptoms after a single dose (effect size 0.31), but the effect leaned on studies with a high risk of bias. It augmented an SSRI in a small randomized trial of OCD. In anxiety, the results are inconsistent, though one well-designed trial found a benefit for anxiety in schizophrenia-spectrum patients (effect size 0.54, 400 mg/day for 8 weeks).
For depression, the authors found a moderate antidepressant signal (effect size 0.69) once one outlier study was dropped, but that rests on just three small trials and needs confirmation.
Side Effects
No serious adverse events turned up. Minor complaints like headache, nausea, and dry mouth were no more common than with placebo.
Limitations
Most positive findings come from small, single-dose studies in healthy volunteers, several with a high risk of bias. Only 12 of the 31 trials formally tracked side effects.
Practice Implications
- L-theanine has acute and sustained benefits for cognition, much like its parent drink, tea.
- Small trials suggest benefits in depression, OCD, and schizophrenia, but these need replication.
- Here are some lab-tested products.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
