L-theanine

Most trials test L-theanine either on its own or together with caffeine, and the grades below keep the two apart. Our evidence grading method explains what each grade requires, and the evidence matrix sets L-theanine beside other compounds. Related goal hubs cover focus, mood and stress and sleep.

A note on the numbers: a meta-analysis pools results from several trials, and many of these papers report a standardized mean difference (SMD), an effect size that expresses the gap between groups in standard units. A randomized controlled trial (RCT) assigns people to the supplement or a placebo by chance.

Does L-theanine with caffeine improve attention switching?

In short-term trials, yes, though much of the effect may come from the caffeine. A 2014 meta-analysis of 11 randomized placebo-controlled studies found moderate effect sizes favoring combined caffeine and L-theanine for attention-switching accuracy and self-rated alertness in the first 2 hours after a dose [3]. Its moderator analysis found trends toward larger changes in effect size with caffeine dose than with L-theanine dose, particularly in the first hour [3].

A 2025 meta-analysis of trials in healthy participants, co-funded by Lipton Teas and Infusions and with authors employed by Lipton and Unilever, found that L-theanine plus caffeine improved attention-switching accuracy in the second hour compared with placebo (SMD 0.33; 95% CI 0.13 to 0.54) [2]. The same industry co-funded review also found better digit vigilance accuracy in the second hour (SMD 0.20; 95% CI 0.02 to 0.38) [2]. The industry co-funded review's authors noted that the confidence intervals frequently showed uncertainty about the direction and size of these differences [2].

The individual trials behind this outcome are small, and the three described here were each led by Unilever researchers. In a placebo-controlled trial led by Unilever researchers in 44 young adults, 97 mg L-theanine with 40 mg caffeine improved accuracy during task switching and self-reported alertness, but had no significant effect on visual search, choice reaction time or mental rotation [4]. A second Unilever-led crossover trial using the same doses found better attention on a switch task compared with placebo, while subjective alertness did not improve significantly [5]. In a third trial led by Unilever researchers in 27 participants, 100 mg L-theanine with 50 mg caffeine improved both speed and accuracy on an attention-switching task at 60 minutes [6].

Trials using higher doses found related benefits on other tasks. In a crossover trial of 250 mg L-theanine and 150 mg caffeine, the combination led to faster simple reaction time, faster numeric working memory reaction time and better sentence verification accuracy [7].

Does L-theanine on its own improve reaction time?

Pooled data point to a short-term benefit, but single trials disagree. A 2026 meta-analysis of 31 randomized placebo-controlled trials (1,168 participants, from healthy and clinical populations) found that a single 200 mg dose taken 30-60 minutes before testing improved choice reaction time in healthy adults (SMD 0.51; 95% CI 0.25 to 0.77) [1]. One of its four authors founded a company that sells dietary supplements, according to the paper's competing-interest statement [1].

The 2025 meta-analysis co-funded by Lipton Teas and Infusions also found a small-to-moderate difference in choice reaction time in the first hour that favored L-theanine alone over placebo (SMD -0.35; 95% CI -0.61 to -0.10) [2].

Evidence from single trials is mixed. In a five-way crossover trial in 20 healthy men, 200 mg L-theanine, 160 mg caffeine and their combination each significantly improved recognition visual reaction time, while placebo did not [9]. That trial found no significant interaction between theanine and caffeine on this measure [9]. By contrast, in a trial in 48 healthy adults, 200 mg theanine slowed overall reaction time on a visual probe task [8]. The trials used different tests and doses, so the pooled estimates from the 2026 meta-analysis and the Lipton co-funded 2025 meta-analysis are the better guide [1][2]. The 2026 estimate is for a single 200 mg dose [1].

Does L-theanine reduce perceived stress?

Evidence is mixed, and the pooled effect leans on weaker studies [1]. In the 2026 meta-analysis, the reduction in acute stress after a single dose in healthy adults was modest (SMD 0.31) and largely influenced by studies with a high risk of bias, meaning studies whose design leaves more room for error [1]. A 2020 systematic review of 9 randomized controlled trials concluded that 200-400 mg/day may help reduce stress in people exposed to stressful conditions, and called for longer-term and larger studies [10].

In a small double-blind crossover study of 12 participants, L-theanine reduced heart rate and salivary immunoglobulin A responses to an acute stress task compared with the placebo condition [11]. These are physical stress markers measured in a laboratory, not everyday stress.

The longer trial cited here is small and was funded by a supplement company. In a 28-day RCT in 30 healthy adults with moderate stress, funded by the supplement company Ethical Naturals, which employed one author, perceived stress scores fell by 17.98% with 400 mg/day of the branded ingredient AlphaWave L-theanine and by 17.88% with placebo [13]. In that Ethical Naturals-funded trial, there was no statistically significant between-group difference in change in salivary cortisol [13]. When the placebo group improves by almost the same amount, the trial does not show a benefit of the supplement itself.

Does L-theanine improve sleep quality?

Possibly, on self-rated measures, but the trials are mixed in design and population. A 2025 meta-analysis pooled 18 randomized trials (19 articles, 897 participants) in people of all ages and health status [14]. L-theanine improved overall self-rated sleep quality (SMD 0.43; 95% CI 0.04 to 0.83; 12 studies), self-rated time to fall asleep (SMD 0.15; 10 studies) and daytime dysfunction (SMD 0.33; 9 studies) [14]. The authors noted a lack of studies on "pure" L-theanine [14]. Because the review pooled people of all ages and health status [14], its results may not carry over to healthy adults.

A 2026 systematic review of 13 trials (550 participants; 11 RCTs and 2 single-arm open-label trials), written by researchers at British American Tobacco's research and development unit and its subsidiary The Water Street Collective, reported beneficial effects on both objective and self-reported outcomes [15]. That industry-authored review covered doses of 50-900 mg/day [15].

Trials in healthy adults are small and supplier-funded. In a 4-week crossover trial in 30 healthy adults, funded by the ingredient supplier Taiyo Kagaku, which supplied the tablets and employed two authors, 200 mg/day L-theanine reduced sleep-questionnaire scores for sleep latency, sleep disturbance and use of sleep medication compared with placebo [12]. In the Ethical Naturals-funded 28-day AlphaWave trial, however, the L-theanine group showed decreased time asleep and less light sleep compared with placebo [13]. An NIH-funded 2025 review describes sleep findings for L-theanine as often inconsistent [17].

What the evidence does not show

The trials do not show how much of the combined effect comes from L-theanine rather than caffeine: in the 2014 meta-analysis, a moderator analysis found trends toward larger changes in effect size with caffeine dose than with L-theanine dose [3]. In the trial that measured it, adding 200 mg theanine to 250 mg caffeine did not significantly change jitteriness, alertness or other aspects of mood [8]. The 2026 meta-analysis found no significant effect on fatigue [1].

Long-term benefit is not established. A 2025 review funded by the US National Institutes of Health concluded that the evidence for many health claims remains limited, especially because of a lack of rigorous human clinical trials [17], and MSKCC states that data in humans are limited [24]. The 2025 Lipton co-funded meta-analysis also called for research using tea-equivalent doses and in free-living participants [2].

In a randomized crossover study in 12 healthy participants, blood levels of L-theanine peaked 0.8 hours after a 100 mg capsule [16]. Uptake and urinary excretion were comparable whether the 100 mg came from capsules or from green tea [16]. The study authors concluded that L-theanine is rapidly absorbed and seems to be broken down into ethylamine and glutamic acid [16].

Timing in trials followed this pattern. The single-dose trials pooled in the 2026 meta-analysis tested people 30-60 minutes after a 200 mg dose [1], and the combined effects with caffeine in the 2014 meta-analysis were seen within the first 2 hours [3]. These describe when trials measured effects, not when anyone should take it.

Health Canada's monograph covers L-theanine isolated from tea leaves or made synthetically [22].

Safety data from trials are limited [1]. The 2026 meta-analysis, which compared dropout rates and reasons between L-theanine and placebo groups, reported no serious adverse events [1]. In one crossover trial, a single 250 mg dose of L-theanine increased self-rated headache and reduced correct answers on a serial subtraction task [7]. In the Ethical Naturals-funded 28-day trial of 400 mg/day AlphaWave L-theanine, all adverse events had resolved by the end of the study or at follow-up [13]. MSKCC states that side effects have not been reported [24].

Reviews describe the toxicology as reassuring but incomplete. A 2017 review concluded from acute and chronic toxicity tests that L-theanine is reliable in general even at high dietary intakes, while noting that no clear evidence-based conclusion on its metabolism, health effects and safety had yet been reached [18]. The NIH-funded 2025 review describes a good safety profile based on toxicology studies, but urges caution in the use of L-theanine supplements at pharmacologic doses by the wider population [17].

In a 13-week animal study in rats of the branded ingredient Suntheanine, doses went up to 4000 mg/kg of body weight a day, and the no-observed-adverse-effect level was the highest dose tested [19]. Animal results do not set a safe dose for people.

The figures regulators cite are conditions of use rather than upper limits: up to 250 mg per serving in the US GRAS notices for foods [20][21], and 200-250 mg a day for adults in Health Canada's monograph [22].

As of November 11, 2026, the sources we checked (the LiverTox database, and the NIH Office of Dietary Supplements fact sheet list as archived on November 1, 2026) do not include an entry for L-theanine.

Interaction evidence is thin. MSKCC reports that taking L-theanine with sedatives and sleep medicines, such as lorazepam, diazepam or zolpidem, may increase drowsiness, and it lists sedatives as a contraindication [24]. MSKCC also reports that L-theanine had synergistic or additive effects with midazolam in animal models, with clinical relevance yet to be determined [24]. According to MSKCC, L-theanine does not induce or inhibit CYP450 enzymes [24].

MSKCC advises people having chemotherapy to discuss L-theanine with their physicians [24]. With caffeine, one placebo-controlled trial found that 200 mg theanine counteracted the rise in blood pressure caused by 250 mg caffeine [8].

As of November 11, 2026, the sources we checked (PubMed searches run on November 11, 2026, and MSKCC [24]) do not include a human study designed to test interactions between L-theanine and prescription medicines, including the sedatives and sleep medicines named above. The blood-pressure finding above comes from a trial of L-theanine with caffeine, not with a medicine [8].

If you take any of these medicines, talk to your doctor or pharmacist before combining them with L-theanine.