Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials.
Cyril Gerolymos, Elisa Saddier, Laurent Boyer et al.
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In 30 seconds
This systematic review and meta-analysis pooled 31 randomised controlled trials (n = 1168) comparing oral L-theanine with placebo in healthy and clinical populations. The clearest result was improved choice reaction time after a single 200 mg dose taken 30-60 min before cognitive testing (SMD = 0.51; 95% CI, 0.25-0.77), consistent with better short-term attention. Acute stress reduction was modest and influenced by studies at high risk of bias; fatigue did not improve significantly. A possible reduction in depressive symptoms requires confirmation, and no serious adverse events were reported.
Key findings
- A single 200 mg dose improved choice reaction time (SMD = 0.51; 95% CI, 0.25-0.77).
- Acute stress reduction was modest (SMD = 0.31) and largely influenced by studies with a high risk of bias.
- After excluding one outlier, a single dose reduced depressive symptoms in healthy individuals (SMD = 0.69; 95% CI, 0.13-1.25).
- No significant effect on fatigue or serious adverse events was reported; anxiety findings were inconsistent and non-significant apart from one study.
Why it matters
The findings distinguish a short-term attention signal from less certain effects on stress and mood. This may inform evaluation of L-theanine as a cognitive or affective intervention, while the evidence does not establish broad clinical benefit. The possible depressive-symptom effect is particularly uncertain because it was observed in healthy individuals and the authors call for confirmation in high-quality trials, especially in clinical populations.
What to keep in mind
The acute stress result was largely influenced by studies at high risk of bias. The potential depressive-symptom effect followed exclusion of one outlier and needs confirmation, particularly in clinical populations.