L-Theanine

• 200 mg/dn = 30 · 4 weeks
• p = 0.006trait anxiety ↓
• p = 0.013sleep quality (PSQI) ↓

A 4-week randomized, placebo-controlled crossover trial in 30 adults without major psychiatric illness found that 200 mg/day L-theanine significantly reduced self-rated depression, trait anxiety, and overall sleep-quality scores versus placebo. This is one of the cleaner direct-monotherapy human RCTs supporting the stress-reduction use case.

Reported effect: n=30 (age 48.3 ± 11.9 yr), 200 mg/day for 4 weeks; Self-rating Depression Scale, STAI-trait anxiety, and PSQI scores all decreased after L-theanine (p = 0.019, 0.006, and 0.013 respectively)

“Self-rating Depression Scale, State-Trait Anxiety Inventory-trait, and Pittsburgh Sleep Quality Index (PSQI) scores decreased after L-theanine administration (p = 0.019, 0.006, and 0.013, respectively). ... L-theanine (200 mg/day) or placebo tablets were randomly and blindly assigned for four-week administration.”

Source: PMID 31623400 · Hidese et al. 2019 · Nutrients

• 9 RCTscontrolled studies reviewed
• 200-400 mg/dsuggested range

A systematic review of human RCTs concluded that 200-400 mg/day L-theanine may help reduce stress and anxiety in people exposed to stressful conditions, while explicitly cautioning that larger, longer-term trials are still needed before it can be justified as a therapeutic agent. The review reported the scope of evidence (9 controlled studies) but did not pool a quantitative effect size.

Reported effect: 9 controlled RCTs reviewed; suggested dose range 200-400 mg/day; no pooled effect size reported in abstract

“9 peer-reviewed journal articles were identified where L-THE as a supplement was compared to a control ... supplementation of 200-400 mg/day of L-THE may assist in the reduction of stress and anxiety in people exposed to stressful conditions ... longer-term and larger cohort clinical studies ... are needed to clinically justify the use of L-THE as a therapeutic agent”

Source: PMID 31758301 · Williams et al. 2020 · Plant Foods Hum Nutr

• n = 12crossover RCT
• HR ↓ · s-IgA ↓vs placebo (no values reported)

In a classic crossover RCT of 12 participants, L-theanine intake reduced the heart-rate and salivary IgA rise triggered by an acute stress task versus placebo, with heart-rate variability analysis suggesting attenuated sympathetic nervous activation. The abstract reports the direction of effect but no specific numerical values, p-values, or dose, so no effect size could be extracted.

Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.

“The results showed that L-Theanine intake resulted in a reduction in the heart rate (HR) and salivary immunoglobulin A (s-IgA) responses to an acute stress task relative to the placebo control condition ... analyses of heart rate variability indicated that the reductions in HR and s-IgA were likely attributable to an attenuation of sympathetic nervous activation”

Source: PMID 16930802 · Kimura et al. 2007 · Biol Psychol

• +38.1 msreaction time vs placebo · p=0.003
• P = 0.02attention hit rate ↑
• n = 37200 mg + 160 mg caffeine

A double-blind, placebo-controlled crossover RCT in 37 overnight sleep-deprived young adults found a 200 mg L-theanine + 160 mg caffeine combination significantly improved selective-attention accuracy and reaction time and enhanced P3b ERP amplitude/latency versus placebo, supporting the caffeine-synergy use case for focus under fatigue.

Reported effect: n=37 (age 22-30), 200 mg L-theanine + 160 mg caffeine; improved hit rate (P=0.02) and target-distractor discriminability (P=0.047); reaction-time improvement to accident scenes Δ=52.08 ms (P<0.0001) vs placebo Δ=13.97 ms (P=0.024), between-group difference Δ=38.1 ms (P=0.003)

“L-theanine-caffeine combination significantly improved the hit rate (P = 0·02) ... target-distractor discriminability (P = 0·047) ... △ = 52·08 ms, P < 0·0001 ... pre-post-dose reaction time improvement ... significantly greater than ... placebo (△ = 38·1 ms, P = 0·003) ... Thirty-seven overnight sleep-deprived healthy adults (aged 22-30 years, twenty-one men)”

Source: PMID 40789769 · Nawarathna et al. 2025 · Br J Nutr

• ages 50-69single-dose RCT
• RT ↓ · errors ↓attention + working memory (no values)

A double-blind, randomized, placebo-controlled study in Japanese adults aged 50-69 found a single dose of L-theanine reduced reaction time on an attention task (Stroop Part 1) and increased correct answers while decreasing omission errors on a working-memory task. The abstract reports directional cognitive improvements but does not give sample size, dose, or p-values, so no numeric effect size could be extracted.

Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.

“The single dose of l-theanine reduced the reaction time to attention tasks (Stroop test, Part 1) ... it increased the number of correct answers and decreased the number of omission errors in working memory tasks (4-Part continuous performance test, Part 4) ... The subjects were Japanese men and women aged 50-69 years.”

Source: PMID 33751906 · Baba et al. 2021 · J Med Food

• p = 0.01HDRS reduction at week 6
• 200 mg/d+ sertraline 100 mg/d
• n = 60MDD patients

A randomized, double-blind, placebo-controlled trial added 200 mg/day L-theanine to sertraline in 60 MDD patients (50 completers). The L-theanine arm showed greater reductions in Hamilton Depression Rating Scale scores at weeks 2, 4, and 6 and higher response and remission rates at week 6, with side-effect frequency comparable to placebo. This is a clinical-population adjunct finding, not evidence for L-theanine as a standalone depression treatment.

Reported effect: n=60 enrolled / 50 completers (25/group), sertraline 100 mg/d + L-theanine 200 mg/d; greater HDRS reduction at weeks 2/4/6 (p = 0.02, 0.03, 0.01); superior response and remission at week 6 (p = 0.05 and 0.02)

“A greater reduction in HDRS scores was observed in the l-theanine group from baseline to weeks 2, 4, and 6 (p-values = 0.02, 0.03, and 0.01, respectively). ... l-theanine was superior to placebo regarding response to treatment and remission rates at week 6 (p-values = 0.05 and 0.02, respectively). ... Sixty MDD (DSM-5) patients ... sertraline (100 mg/d) plus either l-theanine (200 mg/d) or matched placebo.”

Source: PMID 37084960 · Shamabadi & Akhondzadeh 2023 · J Affect Disord

• n = 20elite rowers · 150 mg/6 wk
• Th1/Th2 shiftIL-10 ↓ post-exercise (ratios only)

A small RCT in 20 elite rowers (150 mg/day for 6 weeks) found L-theanine shifted exercise-induced immune markers, notably a post-exercise decrease in anti-inflammatory IL-10 (raising IL-2/IL-10 and IFN-γ/IL-10 ratios) and a post-recovery decrease in cytotoxic lymphocyte counts, interpreted by the authors as a beneficial effect on a disrupted Th1/Th2 balance. The abstract reports directional/ratio changes labeled significant but no numeric effect sizes or p-values.

Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.

“receiving 150 mg of L-theanine extract for 6 weeks, or to the placebo group (n = 10) ... L-theanine contributed to a significant post-exercise decrease in IL-10 concentration, which was reflected by higher values of IL-2 to IL-10 and IFN-γ to IL-10 ratios ... a significant post-recovery decrease in CTL count, Treg to NK and Treg to CTL ratios was observed in the supplemented group ... a beneficial effect on a disrupted Th1/Th2 balance”

Source: PMID 30770758 · Juszkiewicz et al. 2019 · J Int Soc Sports Nutr

• 11 RCTssix countries
• 419 screenedschizophrenia · anxiety · ADHD

A systematic review of 11 RCTs across six countries reported that L-theanine supplementation reduced psychiatric symptoms more than control conditions in schizophrenia, anxiety disorders, and ADHD populations. The review summarized direction of effect across disorders but did not report pooled effect sizes or numeric outcomes, and the findings apply to studied clinical populations rather than to disease treatment claims.

Reported effect: 11 RCTs from six countries (419 publications screened); no pooled effect size or numeric outcome reported in abstract

“Among the 419 publications identified, 11 studies from six countries ... were included in the final analysis. ... LT supplementation significantly reduced psychiatric symptoms more effectively than control conditions in individuals with schizophrenia, anxiety disorders, and ADHD.”

Source: PMID 39633316 · Systematic review 2024 · L-theanine in mental disorders