L-Theanine

Human RCT Supplementscognition sleep Last reviewed 2026-09-28

Summary

An amino acid abundant in tea. Small human trials report increased alpha-band brain activity (a marker of relaxed wakefulness) and improved attention measures when combined with caffeine.

Mechanism

L-theanine is an amino acid structurally related to glutamate that crosses the blood-brain barrier. Human EEG studies associate it with increased alpha-band oscillatory activity, a marker of relaxed alertness rather than sedation. In combination with caffeine, it is reported to improve attention and reduce the subjective jitter some people experience from caffeine, possibly by opposing caffeine's effects on arousal networks.

Reported effects

Ratings are relative to peers in the same category, not absolute claims.

Relaxed wakefulness (alpha activity)★★★☆☆Small EEG trials show increased alpha-band activity vs placebo without drowsiness.

Attention with caffeine★★☆☆☆Combined theanine plus caffeine improved attention measures vs either alone in one RCT.

Stress-related symptoms★★☆☆☆Some support for reduced subjective stress responses; trials are small and short.

Studied dosing: research context

Trials have used 50 mg single doses (alpha-wave studies) up to 250 mg, and 250 mg L-theanine combined with 150 mg caffeine in the cognition/mood RCT. These are doses used in research, not guidance.

Research context only. This is not a recommendation.

Safety

Well tolerated in trials at studied doses. One study in older adults reported more headaches with higher daily doses. Small studies have noted modest blood-pressure lowering, which is relevant for people on antihypertensive medication. Safety data in pregnancy are limited.

Limitations

The evidence base is thin: small, acute single-dose studies with few replications. The Nobre alpha-wave paper is a brief conference supplement abstract. Long-term effects are unstudied, and most attention findings come from theanine-caffeine combinations rather than theanine alone.