L-Theanine

What is L-theanine and what does it do in the body?

According to the review by Türközü and Şanlier, L-theanine is a non-protein amino acid specific to tea; the health effects of tea are mainly associated with catechins and caffeine, while the effects of theanine have come to the fore in recent years. The review says that theanine is known to have favorable effects on relaxation, cognitive performance, mood, and sleep quality, while also stating that no clear evidence-based conclusion has yet been established on theanine’s metabolism, health effects, and safety. ^[8] For this reason, a compound’s “being found in tea” or being described as “relaxing” does not count as evidence of the supplement’s benefit; its benefit is measured separately by controlled studies.

The distinction between L-theanine in tea and a supplement is important: tea as consumed contains caffeine, L-theanine, and catechins together. In EFSA’s evaluation on black tea and attention, tea was treated as a food defined by its caffeine and L-theanine content; the application argued that the effect depended on the combined action of the two substances, but the panel stated that the observed effect could be explained by the caffeine content. ^[9]

Does L-theanine help sleep?

The 2025 meta-analysis by Bulman and colleagues (randomized studies examining L-theanine supplementation in people of all ages and health conditions) evaluated 19 articles (897 participants); 18 entered the meta-analysis. L-theanine significantly improved subjective sleep onset latency (SMD 0.15; 95% CI 0.01–0.29; 10 studies), subjective daytime dysfunction (SMD 0.33; 95% CI 0.16–0.49; 9 studies), and overall subjective sleep quality score (SMD 0.43; 95% CI 0.04–0.83; 12 studies). The authors state that the lack of “pure” L-theanine studies calls for further research and that more studies are needed on the appropriate dose and duration. ^[1] Effect sizes are small, and the outcomes are subjective sleep questionnaires.

In the crossover study by Hidese and colleagues, 30 healthy adults (48.3±11.9 years; 21 women) received 200 mg/day of L-theanine or placebo for 4 weeks; the Pittsburgh Sleep Quality Index and the sleep onset latency, sleep disturbance, and use of sleep medication subscores decreased with L-theanine. The study was funded by a manufacturer of supplement raw materials that supplied the L-theanine and placebo tablets, and the authors include employees of this company; the sample is small. ^[4]

Does L-theanine help attention and cognitive function?

In the meta-analysis by Gerolymos and colleagues covering 31 randomized studies (n=1,168), a single 200 mg dose taken 30–60 minutes before the cognitive test improved choice reaction time (SMD 0.51; 95% CI 0.25–0.77); the authors interpreted this as an improvement in attention. There was no significant effect on fatigue. According to the article’s conflict-of-interest statement, one of the authors founded a company that sells dietary supplements. ^[2] In the meta-analysis by Payne and colleagues (50 RCTs; 15 eligible for at least one meta-analysis), in the theanine–placebo comparison, choice reaction time differed in favor of theanine at hour 1 (SMD −0.35; 95% CI −0.61 to −0.10); for the theanine plus caffeine mixture, the figures were more uncertain (choice reaction time hour 1 SMD −0.48; 95% CI −1.01 to 0.05; attention-switching accuracy hour 2 SMD 0.33; 95% CI 0.13–0.54; overall mood hour 2 SMD 0.26; 95% CI −0.10 to 0.63). The authors state that the confidence intervals often show uncertainty about the direction and size of the effect; some of the authors work at the research unit of a tea company and at the innovation center of a food company. ^[3]

In EFSA’s 2018 evaluation, a cause-and-effect relationship was established between black tea consumption and improvement in attention, but the panel was of the view that the effect could be explained by the caffeine content, and the wording the panel accepted was “Owing to its caffeine content, black tea improves attention”; it was stated that 2–3 servings of black tea providing a total of at least 75 mg of caffeine within 90 minutes are needed for the effect to be seen. The application came from a food company. ^[9]

The panel stated that the definition of black tea (tea solids, caffeine, and L-theanine content) was sufficient, that the three human intervention studies in the application showed an effect of black tea on attention, that the application attributed the effect to the combined action of caffeine and L-theanine, and that the panel found the effect explainable by the caffeine content. ^[9]

Does L-theanine reduce stress and anxiety?

In the meta-analysis by Gerolymos and colleagues, the primary outcome was the effect of a single dose on acute stress in healthy adults; the reduction was small (SMD 0.31) and was mostly influenced by studies at high risk of bias. When one outlier study was removed, a significant reduction in depressive symptoms was found in healthy people after a single dose (SMD 0.69; 95% CI 0.13–1.25); anxiety effects were inconsistent and not significant apart from a single study on psychotic anxiety (400 mg/day, 8 weeks; SMD 0.54). No serious adverse events were reported. ^[2] In the Hidese study, 200 mg/day of L-theanine over 4 weeks reduced scores on the Self-rating Depression Scale and the State-Trait Anxiety Inventory (trait anxiety); verbal fluency and executive function scores improved. ^[4]

In the 8-week double-blind study by Sarris and colleagues in 46 patients with generalized anxiety disorder (DSM-5 diagnosis, on a stable antidepressant), 450–900 mg/day of add-on L-theanine did not surpass placebo on the HAMA anxiety score (p=0.73) and the Insomnia Severity Index (p=0.35); there was a difference in favor of L-theanine on the sleep satisfaction item (p=0.015) and in those with non-clinical insomnia levels (ISI ≤14; p=0.007); there was no cognitive effect. The authors stated that the study did not support efficacy for anxiety in GAD. ^[5]

The systematic review by Moshfeghinia and colleagues covers 11 randomized studies from 6 countries (schizophrenia, ADHD, OCD, major depression, sleep disorders, GAD, Tourette syndrome); it reports that L-theanine reduced symptoms more than control in schizophrenia, anxiety disorders, and ADHD; the authors state that more studies are needed to confirm the results. ^[7] While the review reports a favorable result for anxiety disorders, no anxiety effect was found in the Sarris GAD study examined on its own; the evidence is therefore inconsistent.

Is L-theanine effective in children?

In the study by Lyon and colleagues, 98 boys aged 8–12 with diagnosed ADHD (stratified by stimulant medication use) received two chewable tablets (100 mg each; 400 mg/day total) of L-theanine or placebo twice daily for 6 weeks. On actigraphy over five nights, sleep percentage and sleep efficiency were significantly higher in the L-theanine group; sleep onset latency and other parameters did not change; the Pediatric Sleep Questionnaire completed by parents did not agree with the objective data. No serious adverse events were reported. A commercially branded product was used in the study, and the product’s manufacturer is named in the article. ^[6] Because it is a single study, no general conclusion can be drawn for children.

How should L-theanine studies be read?

Four common features stand out in L-theanine studies. First, most outcomes are subjective questionnaires (sleep, stress, mood), and effect sizes are between small and moderate. ^{[1] [2]} Second, in objective measures such as attention and reaction time, the effect is short-lived and measured in the hours after a single dose; in the Gerolymos analysis, the primary outcome is the acute effect of a single dose. ^[2] Third, many studies use products in which L-theanine is given with caffeine or other components; Bulman and colleagues explicitly state the lack of pure L-theanine studies, while Payne and colleagues analyze theanine plus caffeine mixtures separately. ^{[1] [3]} Fourth, some of the favorable findings come from groups linked to the study authors or the product’s manufacturer. ^{[3] [4] [6]} These features do not show that the results are wrong; they explain the uncertainty about the size and generalizability of the effect.

Is L-theanine safe?

In the meta-analysis by Gerolymos and colleagues, no serious adverse events were reported; dropout rates and reasons were compared between L-theanine and placebo. ^[2] The review by Türközü and Şanlier states that, based on acute and chronic toxicity studies, L-theanine is generally safe even when taken in high amounts through the diet; however, no clear evidence-based conclusion on metabolism and safety has yet emerged. ^[8] The 450–900 mg/day used in the GAD study is above the 300 mg/day upper limit in Türkiye. ^{[5] [10]}

In caffeinated mixed products, which component produces the effect cannot be separated out: EFSA attributed the effect in black tea to caffeine, while the Payne meta-analysis analyzed theanine plus caffeine mixtures separately. ^{[9] [3]}

How is L-theanine regulated in Türkiye?

In Türkiye, food supplements are regulated by the Ministry of Agriculture and Forestry, and Ministry approval is mandatory for every food supplement to be placed on the market. ^[12] Two separate Ministry lists are relevant to this topic:

• Restricted Substances List (last updated September 21, 2026): In the L-theanine row, the source is “Green Tea (Camellia sinensis)”, the 11 years and older daily intake dose column is 300 mg/day, and the publication date is June 27, 2016; the ages 4–10 column is empty. ^[10] This limit is for the amount of L-theanine; it is not in the same unit as caffeine or tea extract amounts.
• Plant List (updated July 16, 2026): Camellia sinensis leaf (“Tea / Green Tea”) appears in the list as positive (P). ^[11]

The amounts used in studies (a single 200 mg dose, 200 mg/day for 4 weeks, 400 mg/day in children, 450–900 mg/day in GAD) and the upper limit in the list (300 mg/day) therefore cannot be assessed within the same framework; the list is an upper limit, not evidence of benefit. ^{[4] [6] [5] [10]}

Evidence table: claim, population, and limits