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L-theanine and perceived stress: grade C

Limited evidence. Direction: Benefit.

C Limited

Educational guidance, not medical advice

Why this grade

Rubric condition met: Pooled effect depends on high risk-of-bias studies; repeated-dose RCTs are small and supplier-funded with mixed results (C, conservative reading per citation-rules 9). Grader to confirm fit: no rubric row describes this pattern exactly.

The 2026 meta-analysis found only a modest reduction in acute stress (SMD 0.31) that was largely driven by high risk-of-bias studies. A systematic review of 9 RCTs judged 200-400 mg a day may help people under stressful conditions but called for longer, larger trials. In a supplier-funded 28-day RCT, perceived stress fell by about the same amount with placebo, and there was no statistically significant between-group difference in change in salivary cortisol. Evidence is mixed.

How we grade evidence

Studies this grade rests on

EffectMixedNo effectSize shows participants (n)200520102015202020252030Meta-analysisSystematic reviewRCTCrossover RCTGerolymos 2026, 2026: meta-analysis, n = 1,168, result not recordedMoulin 2024, 2024: RCT, n = 30, result not recordedHidese 2019, 2019: crossover RCT, n = 30, result not recordedKimura 2007, 2007: crossover RCT, n = 12, result not recordedWilliams 2020, 2020: systematic review, n not recorded, result not recorded
each study this grade rests on, by publication year and design. Solid: effect versus the comparator; half-filled: mixed; hollow: no effect; dashed: result not recorded yet. Larger marks had more participants.
Show the numbers
StudyYearDesignParticipantsResult
Kimura 20072007crossover RCT12result not recorded
Hidese 20192019crossover RCT30result not recorded
Williams 20202020systematic reviewnot recordedresult not recorded
Moulin 20242024RCT30result not recorded
Gerolymos 20262026meta-analysis1168result not recorded

Safety and Efficacy of AlphaWave(®) L-Theanine Supplementation for 28 Days in Healthy Adults with Moderate Stress: A Randomized, Double-Blind, Placebo-Controlled Trial.

Moulin M, Crowley DC, Xiong L. Safety and Efficacy of AlphaWave(®) L-Theanine Supplementation for 28 Days in Healthy Adults with Moderate Stress: A Randomized, Double-Blind, Placebo-Controlled Trial Neurol Ther, 2024. PMID 38758503

Result not recorded
Design
Randomized, double-blind, placebo-controlled trial (28 days)
Sample
n = 30
Population
Healthy adults 18-65 with moderate stress
Funding
Ethical Naturals, Inc. (Crossref funder)

L-Theanine reduces psychological and physiological stress responses.

Kimura K, Ozeki M, Juneja LR. L-Theanine reduces psychological and physiological stress responses Biol Psychol, 2007. PMID 16930802

Result not recorded
Design
Double-blind, counterbalanced placebo-controlled crossover study (acute)
Sample
n = 12
Population
Adults under a laboratory stress task
Funding
not stated in PubMed record

The Effects of Green Tea Amino Acid L-Theanine Consumption on the Ability to Manage Stress and Anxiety Levels: a Systematic Review.

Williams JL, Everett JM, D'Cunha NM. The Effects of Green Tea Amino Acid L-Theanine Consumption on the Ability to Manage Stress and Anxiety Levels: a Systematic Review Plant Foods Hum Nutr, 2020. PMID 31758301

Result not recorded
Design
Systematic review of 9 randomized controlled trials
Population
People exposed to stressful conditions
Funding
not stated in PubMed record

Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial.

Hidese S, Ogawa S, Ota M. Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial Nutrients, 2019. PMID 31623400

Result not recorded
Design
Randomized, placebo-controlled, crossover, double-blind trial (4 weeks)
Sample
n = 30
Population
Adults without major psychiatric illness, mean age 48.3
Funding
Funding received from Taiyo Kagaku Co., Ltd., Mie, Japan., Funding received from Taiyo Kagaku Co., Ltd., Mie, Japan. None

Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials.

Gerolymos C, Saddier E, Boyer L. Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials Mol Psychiatry, 2026. PMID 42410082

Result not recorded
Design
Systematic review and meta-analysis of 31 randomized placebo-controlled trials
Sample
n = 1,168
Population
Healthy and clinical populations (primary outcome: healthy adults, single dose)
Funding
not stated in PubMed record

Search and history

Search (): theanine[ti] AND (stress[tiab] OR relaxation[tiab] OR calm[tiab]) AND (randomized controlled trial[pt] OR clinical trial[pt]) AND humans[mh]

  • Graded C (proposed): Proposed in the monograph draft
  • C to C (approved): Editor sign-off November 13, 2026: grade matches the editor-approved final page