Big effect
A single 200 mg dose of L-theanine (with caffeine) shaved 38 milliseconds off reaction time in sleep-deprived adults — but the combo, not theanine alone, drove the effect.
This well-controlled trial shows a large, statistically reliable improvement in processing speed after one dose, but the effect comes from the caffeine–theanine pairing, the participants were all acutely sleep-deprived, and the broader evidence for theanine alone on reaction time is still weak and inconsistent.
In a double-blind crossover study, sleep-deprived young adults who took 200 mg L-theanine plus caffeine responded 38 ms faster on a selective attention task compared to placebo, with brain-wave measures also indicating sharper neural processing. However, the finding applies only to the combination (not theanine by itself) and only under acute sleep loss; the overall research base for theanine’s effect on reaction time is limited and mixed.
Where this fits in the evidence
Pillser has synthesized 3 studies on L-Theanine for Reduced Reaction Time — overall evidence strength: Low.
Across 3 studies, 2 reported beneficial effects of L-theanine on reduced reaction time, with effect sizes ranging from small to large. The evidence is preliminary and primarily limited to sleep-deprived young adults, with doses of 200 mg showing potential acute benefits. One study reported a neutral finding, noting that L-theanine did not significantly differ from placebo despite within-group improvements.
The study
- Randomized Controlled Trial (RCT)
- 2025-08-12
- The British journal of nutrition
This is a plain-language summary of a research finding, not medical advice. Pillser surfaces research signals to help you decide what's worth investigating — always consult a qualified professional before changing what you take.