Benefits
Sleep quality
A pooled analysis of 18 randomized trials found better self-rated sleep quality (SMD 0.43), slightly faster sleep onset (SMD 0.15) and less daytime dysfunction, but noted few trials of L-theanine alone. In 98 boys with ADHD, 400 mg/day raised measured sleep efficiency, while time to fall asleep did not change.
Relaxation and alpha brain waves
In a small trial of 16 people funded by an ingredient maker, a single 200 mg dose raised alpha brain wave activity, a pattern linked to relaxed wakefulness, more than placebo during a stress test, and lowered salivary cortisol more at one hour. A pooled analysis rated the effect on acute stress as small and mostly from higher-risk studies.
Attention and reaction time
A pooled analysis of 31 trials found a single 200 mg dose taken 30 to 60 minutes before testing improved choice reaction time (SMD 0.51). In 50 Japanese adults aged 50 to 69, one 100.6 mg dose sped up attention tasks, but 12 weeks of daily use did not change attention.
Stress and anxiety
Results are mixed. In a 28-day trial in 30 stressed adults, perceived stress fell by about 18% on both L-theanine and placebo, and cortisol did not differ. In 46 adults with generalized anxiety disorder on antidepressants, 450 to 900 mg/day showed no significant effect on anxiety (p = 0.73). Pooled anxiety results were inconsistent.
Mood and emotional well-being
A pooled analysis found lower depressive symptom scores after a single dose in healthy people once one outlier study was removed. In a small trial of 60 adults with major depression taking sertraline, adding 200 mg/day for 6 weeks lowered depression scores more than placebo. It has not been tested as a stand-alone option for diagnosed depression.
Mechanism of action
Alpha brain wave activity
In a crossover trial in moderately stressed adults, a single 200 mg dose raised frontal alpha power on EEG more than placebo 3 hours after the dose. Alpha activity is a brain measurement linked to relaxed wakefulness, not a clinical outcome.
Stress hormone response
After a single 200 mg dose, salivary cortisol fell more than with placebo one hour later during an arithmetic stress test, but 28 days of 400 mg/day did not change cortisol compared with placebo.
Reaches the bloodstream
In a 4-week trial of 200 mg/day, serum L-theanine rose after supplementation, confirming it is absorbed. How it affects the brain in people has not been directly measured in these trials.
Clinical trials
Randomized, double-blind, placebo-controlled trial of L-theanine 450 to 900 mg/day for 8 weeks added to stable antidepressant treatment (Sarris et al. 2019, J Psychiatr Res).
46 adults with DSM-5 generalized anxiety disorder.
L-theanine did not beat placebo on the Hamilton anxiety scale (p = 0.73) or the Insomnia Severity Index (p = 0.35), and cognition did not change. Self-reported sleep satisfaction was better (p = 0.015), as was the insomnia index in people without clinical insomnia (p = 0.007).
Randomized, double-blind, placebo-controlled crossover trial of L-theanine 200 mg/day for 4 weeks, funded by the ingredient maker Taiyo Kagaku (Hidese et al. 2019, Nutrients).
30 healthy Japanese adults (9 men, 21 women; mean age 48) without major psychiatric illness.
Depression, trait anxiety and sleep quality (PSQI) scores improved from baseline during L-theanine but not placebo. Compared directly with placebo, only the sleep latency, sleep disturbance and sleep medication subscales improved more (p about 0.05); total PSQI (p = 0.073), depression (p = 0.084), trait anxiety (p = 0.13) and cognition did not differ.
Randomized, double-blind, placebo-controlled trial of Suntheanine L-theanine 400 mg/day (two 100 mg chewable tablets twice daily) for 6 weeks, with wrist actigraphy (Lyon et al. 2011, Altern Med Rev).
98 boys aged 8 to 12 with diagnosed ADHD, stratified by stimulant use.
Sleep percentage and sleep efficiency were higher than with placebo. Time to fall asleep and the other sleep measures did not change, and parent questionnaires did not match the actigraphy. No significant adverse events.
Randomized, triple-blind, placebo-controlled crossover trial of a single 200 mg dose, funded by Ethical Naturals (Evans et al. 2021, Neurol Ther).
16 healthy adults with moderate stress (one dropped out after the first period).
Frontal and whole-scalp alpha power rose more than with placebo 3 hours after the dose (p 0.050 or less), and salivary cortisol fell more at one hour (p < 0.001). It was well tolerated.
Randomized, double-blind, placebo-controlled trial of 400 mg/day for 28 days, funded by Ethical Naturals (Moulin et al. 2024, Neurol Ther).
30 healthy adults aged 18 to 65 with moderate stress, 15 per group.
Perceived stress fell 17.98% on L-theanine and 17.88% on placebo, and cortisol did not differ between groups. Light sleep fell more than with placebo (p 0.040 or less), and Stroop reaction time improved by day 14, while placebo improved only by day 28.
Randomized, double-blind, placebo-controlled trial of 100.6 mg/day for 12 weeks with a single-dose test, sponsored by ITO EN (Baba et al. 2021, J Med Food).
50 Japanese adults aged 50 to 69 analyzed (26 L-theanine, 24 placebo).
A single dose shortened Stroop reaction time and improved working memory accuracy. After 12 weeks of daily use there was no effect on attention.
Eight-week randomized, double-blind, placebo-controlled trial of L-theanine 400 mg/day added to ongoing antipsychotic treatment (Ritsner et al. 2011, J Clin Psychiatry).
60 patients with schizophrenia or schizoaffective disorder; 40 completed.
Anxiety (p = 0.015), positive symptoms and general psychopathology scores fell more than with placebo, with small to moderate effect sizes. Negative symptoms, cognition, functioning and quality of life did not change.
Six-week randomized, double-blind, placebo-controlled trial of L-theanine 200 mg/day added to sertraline 100 mg/day (Shamabadi et al. 2023, J Affect Disord).
60 adults with DSM-5 major depressive disorder; 50 completed, 25 per group.
Hamilton depression scores fell more than with sertraline plus placebo at weeks 2, 4 and 6 (p = 0.02, 0.03 and 0.01), with higher response (p = 0.05) and remission (p = 0.02) at week 6. Side effects were similar. The trial was small and short.