Supplements

L-Theanine

Updated · 22 sources · Editorial policy

L-theanine is an amino acid found almost only in the tea plant, Camellia sinensis. It is not one of the amino acids used to build proteins. It is sold as a dietary supplement and added to foods and drinks, most often for relaxation, stress, sleep and focus, and often paired with caffeine.

The most important fact is that the human evidence is real but modest. Recent meta-analyses find small-to-moderate short-term effects on attention and reaction time and small effects on self-rated sleep, built on many small, often industry-linked trials. It is not an approved drug for any condition, and there is no good evidence that it builds muscle or speeds physical recovery.

This page summarizes what clinical trials, meta-analyses and regulators report. It is for education only and is not medical advice.

Property Detail
Class Non-proteinogenic amino acid from tea; dietary supplement
Chemistry C₇H₁₄N₂O₃, 174.20 g/mol; an N-ethyl derivative of glutamine
Natural sources Tea leaves; mean ≈5.1–6.6 mg per gram of dry white, green, oolong or black tea
Route Oral (capsules, tablets, drinks)
Doses in trials Single doses of 50–250 mg; daily intake of 150–400 mg for 4–8 weeks
Time to peak ≈0.8 h after 100 mg
Half-life About 1 h (LactMed)
US status Dietary supplement; FDA had “no questions” about GRAS use in foods up to 250 mg per serving (GRN 209, 2007)
WADA status Not named on the 2026 Prohibited List

What it is

L-theanine’s chemical name is (2S)-2-amino-5-(ethylamino)-5-oxopentanoic acid, formula C₇H₁₄N₂O₃, molecular weight 174.20 g/mol (PubChem). Put simply, it is glutamine with an ethyl group on the side-chain nitrogen. That makes it a close structural relative of glutamate, the brain’s main excitatory signaling molecule.

Tea is the main dietary source. An analysis of 37 commercial teas found mean L-theanine contents of 6.26, 6.56, 6.09 and 5.13 mg/g in white, green, oolong and black teas, with wide variation within each type and almost none in pu-erh (Boros et al., Pharmacogn Mag 2016). One research group estimated about 20 mg in a cup of black tea (Nobre et al., Asia Pac J Clin Nutr 2008). Supplement doses used in trials (100–400 mg) are therefore several to many cups’ worth.

Proposed mechanism

How L-theanine acts in the brain is not settled. The main proposals come from lab and animal work:

  • Glutamate receptors. In rat brain cells, theanine bound AMPA, kainate and NMDA-type glutamate receptors, but 80 to 30,000 times more weakly than glutamate itself (Kakuda et al., Biosci Biotechnol Biochem 2002). Whether this matters at the levels reached after a normal dose is unknown.
  • Brain-wave changes. In people, 50 mg L-theanine produced a greater rise in resting alpha-band EEG activity than placebo (n = 16 vs 19), a pattern linked to relaxed alertness (Nobre et al., 2008).
  • Metabolites. It is broken down to ethylamine and glutamic acid, so some effects may come from these products rather than from theanine itself (Scheid et al., J Nutr 2012).

Pharmacokinetics

The main human kinetic study was a randomized crossover trial in 12 healthy adults who took 100 mg L-theanine as capsules or as green tea; 3 participants also took 50 and 200 mg capsules (Scheid et al., J Nutr 2012):

Measure Capsules (100 mg) Green tea (100 mg)
Time to peak plasma level 0.8 h 0.8 h
Peak plasma level (mean ± SD) 24.3 ± 5.7 μmol/L 26.5 ± 5.2 μmol/L
Metabolites in plasma and urine Ethylamine, glutamic acid Ethylamine, glutamic acid

Exposure (AUC) rose with dose across 50, 100 and 200 mg, and a small part of the dose was retained in red blood cells. Absorption from tea and from capsules was comparable.

The NIH LactMed database describes the half-life as about 1 hour (LactMed). This short half-life is why trials usually test effects 30–90 minutes after a single dose.

Regulatory status

  • United States. L-theanine is sold as a dietary supplement. Supplements do not need FDA premarket approval; the manufacturer is responsible for safety, and labeled and actual contents can differ (LactMed). As a food ingredient, FDA responded “no questions” in 2007 to a GRAS (generally recognized as safe) notice for L-theanine at up to 250 mg per serving in several food categories (FDA GRN 209).
  • Sport. L-theanine is not named on the WADA 2026 Prohibited List. Combination products can contain other ingredients that are.

What the research shows

Attention and reaction time

This is the most consistent finding, though effects are small to moderate.

Review Scope Key result
Gerolymos et al., Mol Psychiatry 2026 31 RCTs, n = 1,168, healthy and clinical groups A single 200 mg dose taken 30–60 min before testing improved choice reaction time, SMD 0.51 (95% CI 0.25–0.77)
Payne et al., Nutr Rev 2025 50 RCTs of tea, L-theanine or L-theanine plus caffeine in healthy people; 15 pooled L-theanine alone vs placebo: choice reaction time in the first hour, SMD −0.35 (95% CI −0.61 to −0.10)
Mátyus et al., J Clin Med 2025 5 RCTs, n = 148 healthy adults Recognition visual reaction time −15.2 ms (95% CI −28.99 to −1.41); no significant effect on simple reaction time or the Stroop test

SMD (standardized mean difference) expresses the effect in standard-deviation units, so results from different tests can be pooled. Mátyus and colleagues concluded that a benefit was not confirmed across all test methods and called for larger trials with standardized products. Payne and colleagues noted that confidence intervals often left the direction and size of effects uncertain; their review was funded in part by a tea company.

Combined with caffeine

Many trials test L-theanine together with caffeine, the way it occurs in tea. Results are mixed:

  • In a crossover trial, 250 mg L-theanine plus 150 mg caffeine improved several measures, including faster simple reaction time and less mental fatigue. L-theanine alone increased “headache” ratings and lowered serial-subtraction accuracy (Haskell et al., Biol Psychol 2008).
  • At tea-like doses (50 mg L-theanine with 75 mg caffeine) in 24 adults, adding L-theanine cancelled caffeine’s effects on brain blood flow, cognition and mood, and L-theanine alone had no effect (Dodd et al., Psychopharmacology 2015).
  • In the Payne meta-analysis, the combination showed small-to-moderate benefits on attention and mood in the first 2 hours, again with wide confidence intervals.

Stress, anxiety and mood

A 2020 systematic review of 9 RCTs concluded that 200–400 mg/day might help reduce stress and anxiety in people under stressful conditions, while calling for longer and larger trials (Williams et al., Plant Foods Hum Nutr 2020).

The 2026 meta-analysis gives a more cautious picture (Gerolymos et al., 2026):

  • Acute stress: a modest reduction (SMD 0.31) that was largely driven by studies at high risk of bias.
  • Depressive symptoms: a reduction after a single dose in healthy people, SMD 0.69 (95% CI 0.13–1.25), but only after excluding one outlying study.
  • Anxiety: inconsistent and not significant overall.
  • Fatigue: no significant effect.

The best-known longer trial was a 4-week randomized, double-blind crossover study in 30 healthy adults given 200 mg/day (Hidese et al., Nutrients 2019). Depression, trait-anxiety and sleep-quality scores fell during the L-theanine period. Compared with placebo, the sleep-latency, sleep-disturbance and sleep-medication subscores improved. Two authors worked for the company that supplied the tablets.

Sleep

A 2025 meta-analysis pooled 18 RCTs (897 participants across 19 articles) (Bulman et al., Sleep Med Rev 2025):

Self-reported outcome Studies SMD (95% CI)
Time to fall asleep 10 0.15 (0.01–0.29)
Daytime dysfunction 9 0.33 (0.16–0.49)
Overall sleep quality score 12 0.43 (0.04–0.83)

All three effects favored L-theanine, but they are small and based on questionnaires. The authors noted that few studies used L-theanine on its own, and that the right dose and duration are not known.

In an RCT in 98 boys aged 8–12 with ADHD, 400 mg/day of L-theanine for 6 weeks raised actigraphy-measured sleep percentage and sleep efficiency compared with placebo. Time to fall asleep did not change (Lyon et al., Altern Med Rev 2011).

Psychiatric conditions

In an 8-week randomized, double-blind trial in 60 patients with schizophrenia or schizoaffective disorder (40 completed), 400 mg/day L-theanine added to antipsychotics reduced anxiety and positive symptoms compared with placebo. Effect sizes were small to moderate (Cohen’s d 0.09–0.39), and negative symptoms and cognition did not change (Ritsner et al., J Clin Psychiatry 2011). This has not been established as a treatment.

Exercise, muscle and recovery

We found no trials showing that L-theanine increases muscle mass, strength or physical recovery. The few sports studies point the other way or measure only lab markers:

  • In 20 trained athletes, a single 200 mg dose reduced maximal isometric leg and back strength compared with caffeine and with control, and did not change endurance (Tuncer et al., Front Nutr 2025).
  • In 20 elite rowers, 150 mg/day of an L-theanine extract for 6 weeks shifted some immune markers after maximal exercise (lower IL-10). The study reported immune markers rather than performance outcomes (Juszkiewicz et al., J Int Soc Sports Nutr 2019).

Side effects and risks

Tolerability in trials

Across 31 RCTs, which compared dropout rates and reasons between L-theanine and placebo, no serious adverse events were reported (Gerolymos et al., 2026). Doses of 400 mg/day were described as well tolerated for 6 weeks in boys with ADHD and for 8 weeks in adults with schizophrenia (Lyon et al., 2011; Ritsner et al., 2011). In one acute study, 250 mg increased self-rated headache (Haskell et al., 2008). Most trials were short, so long-term safety in people is not well studied.

Animal toxicology

In a 13-week feeding study in rats, doses up to 4,000 mg/kg/day caused no consistent treatment-related harm; this, the highest dose tested, was the no-observed-adverse-effect level. A higher rate of benign kidney tumors in high-dose females was judged more consistent with genetic predisposition than with a direct toxic effect (Borzelleca et al., Food Chem Toxicol 2006). These are rat data and do not set a human dose.

Interactions

No controlled human interaction studies were found. In mice, injected theanine strengthened the sedative effect of the barbiturate pentobarbital (Yu et al., Drug Chem Toxicol 2016). Whether this applies to people taking sedatives is unknown. In humans, L-theanine also changed the effects of caffeine in some trials (see above).

Pregnancy and breastfeeding

There is no information on L-theanine supplements during breastfeeding. LactMed considers the amounts in green tea likely acceptable, but says high-dose supplements are not necessarily safe for the infant and that it is probably best to avoid them while nursing a newborn or preterm infant (LactMed).

Key takeaways

  • A tea amino acid, not a drug. L-theanine is a glutamine relative found in tea. It is sold as a supplement, and FDA has accepted GRAS use in foods up to 250 mg per serving.
  • Fast in, fast out. Blood levels peak about 0.8 h after a dose, and the half-life is about 1 hour.
  • Small effects on attention. Meta-analyses find small-to-moderate short-term gains in reaction time after about 200 mg, but not on every cognitive test.
  • Modest, uncertain effects on stress and sleep. Self-rated sleep and stress improve slightly in pooled trials, but many studies are small, short or industry-linked.
  • No muscle or recovery benefit shown. Sports trials do not support anabolic or recovery claims, and one found lower strength after a single dose.
  • Well tolerated in short trials. No serious adverse events were reported in trials of up to 8 weeks; long-term human data are limited.

Sources

  1. National Center for Biotechnology Information. L-Theanine, PubChem CID 439378. PubChem.
  2. US Food and Drug Administration. GRAS Notice No. 209: L-theanine (Taiyo International). Closed February 5, 2007; FDA has no questions. FDA.
  3. Drugs and Lactation Database (LactMed). Theanine. National Institute of Child Health and Human Development; updated May 15, 2025. PubMed.
  4. World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.
  5. Scheid L, Ellinger S, Alteheld B, et al. Kinetics of L-theanine uptake and metabolism in healthy participants are comparable after ingestion of L-theanine via capsules and green tea. J Nutr. 2012;142(12):2091–2096. PubMed.
  6. Kakuda T, Nozawa A, Sugimoto A, Niino H. Inhibition by theanine of binding of [3H]AMPA, [3H]kainate, and [3H]MDL 105,519 to glutamate receptors. Biosci Biotechnol Biochem. 2002;66(12):2683–2686. PubMed.
  7. Boros K, Jedlinszki N, Csupor D. Theanine and caffeine content of infusions prepared from commercial tea samples. Pharmacogn Mag. 2016;12(45):75–79. PubMed.
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  9. Gerolymos C, Saddier E, Boyer L, Fond G. Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials. Mol Psychiatry. Published online July 6, 2026. doi:10.1038/s41380-026-03727-9. PubMed.
  10. Payne ER, Aceves-Martins M, Dubost J, et al. Effects of tea (Camellia sinensis) or its bioactive compounds l-theanine or l-theanine plus caffeine on cognition, sleep, and mood in healthy participants: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2025;83(10):1873–1891. PubMed.
  11. Mátyus RO, Szikora Z, Bodó D, et al. Promising, but not completely conclusive—the effect of l-theanine on cognitive performance based on the systematic review and meta-analysis of randomized placebo-controlled clinical trials. J Clin Med. 2025;14(21):7710. PubMed.
  12. Haskell CF, Kennedy DO, Milne AL, et al. The effects of L-theanine, caffeine and their combination on cognition and mood. Biol Psychol. 2008;77(2):113–122. PubMed.
  13. Dodd FL, Kennedy DO, Riby LM, Haskell-Ramsay CF. A double-blind, placebo-controlled study evaluating the effects of caffeine and L-theanine both alone and in combination on cerebral blood flow, cognition and mood. Psychopharmacology (Berl). 2015;232(14):2563–2576. PubMed.
  14. Williams JL, Everett JM, D’Cunha NM, et al. 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;75(1):12–23. PubMed.
  15. Hidese S, Ogawa S, Ota M, et al. Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomized controlled trial. Nutrients. 2019;11(10):2362. PubMed.
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  17. Lyon MR, Kapoor MP, Juneja LR. The effects of L-theanine (Suntheanine) on objective sleep quality in boys with attention deficit hyperactivity disorder (ADHD): a randomized, double-blind, placebo-controlled clinical trial. Altern Med Rev. 2011;16(4):348–354. PubMed.
  18. Ritsner MS, Miodownik C, Ratner Y, et al. L-theanine relieves positive, activation, and anxiety symptoms in patients with schizophrenia and schizoaffective disorder: an 8-week, randomized, double-blind, placebo-controlled, 2-center study. J Clin Psychiatry. 2011;72(1):34–42. PubMed.
  19. Tuncer SY, Ozdenk S, Yildirim UC, et al. Acute effects of combined and isolated caffeine and theanine supplementation on physical and cognitive performance in competitive athletes: a randomized, double-blind, placebo-controlled crossover study. Front Nutr. 2025;12:1751673. PubMed.
  20. Juszkiewicz A, Glapa A, Basta P, et al. The effect of L-theanine supplementation on the immune system of athletes exposed to strenuous physical exercise. J Int Soc Sports Nutr. 2019;16(1):7. PubMed.
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  22. Yu XC, Wu BL, Gao JC, Yang W. Theanine enhanced both the toxicity of strychnine and anticonvulsion of pentobarbital sodium. Drug Chem Toxicol. 2016;39(2):217–223. PubMed.