L-theanine plus caffeine improved attention within two hours, meta-analysis finds

A combination of L-theanine and caffeine produced small-to-moderate improvements in attention and mood within two hours of ingestion, a meta-analysis of 15 randomized controlled trials has found. The strongest result was for attention switching accuracy at the two-hour mark (SMD 0.33, 95% CI 0.13 to 0.54). L-theanine taken alone improved choice reaction time within the first hour (SMD -0.35, 95% CI -0.61 to -0.10), but did not separate from placebo on any other measure.

The review, led by Edward Payne of the University of Aberdeen’s Rowett Institute, screened 50 RCTs in healthy adults and pooled data from 15 of them. The authors searched Cochrane, Embase, and Ovid Medline through August 2023. The meta-analysis was registered on PROSPERO (CRD42022351601) and published in Nutrition Reviews. Two of the five co-authors, Joy Dubost and Arno Greyling, work for Lipton Teas and Infusions and Unilever respectively, the companies that sell the products under study.

The sample sizes are worth a hard look. Across the 15 meta-analyzed trials, total enrollment was 484 participants, with a median of 27 people per study. Only three trials used parallel-group designs. The other 12 were crossover trials, which extract more statistical power from fewer bodies but introduce carryover-effect risks. The weighted mean age was 32. Older adults, who are more likely to buy cognitive supplements, are not represented in this evidence base.

How the study was designed

Payne and colleagues, including corresponding author Baukje de Roos, used random-effects models with the Knapp and Hartung adjustment, a correction that widens confidence intervals when the number of pooled studies is small. The team assessed risk of bias with the Cochrane RoB 2 tool and split outcomes into two time windows: the first hour (1 to 59 minutes post-intake) and the second hour (60 to 119 minutes).

The exposures fell into three buckets: tea beverages, L-theanine alone, and L-theanine plus caffeine. The third bucket gave the cleanest signal. Tea beverages produced too few eligible trials for meta-analysis. Polyphenols in whole tea may alter how the constituent compounds behave, but the current evidence does not tell us how.

What the combination did

At the two-hour mark, the combination outperformed placebo on three measures. Digit vigilance task accuracy: SMD 0.20 (95% CI 0.02 to 0.38), a small gain. Attention switching accuracy: SMD 0.33 (95% CI 0.13 to 0.54), a bit larger. Overall mood: SMD 0.26 (95% CI -0.10 to 0.63). That last one is important: the confidence interval runs from a small negative effect to a moderate positive one, so the data cannot tell us whether the mood benefit is real or noise.

Choice reaction time at one hour followed the same pattern. The point estimate looked decent (SMD -0.48) but the confidence interval stretched from a large benefit to essentially zero (-1.01 to 0.05). Every outcome in this review told a version of this story: L-theanine plus caffeine pointed in the right direction, but the estimates were too imprecise to be sure.

A 2008 crossover trial by Haskell and colleagues, published in Biological Psychology, established the basic template for this research. Twenty-seven healthy adults. Fifty milligrams of caffeine plus 100 mg of L-theanine. The combination improved digit vigilance reaction time and cut self-reported tiredness versus placebo. The Payne meta-analysis pools 15 such trials and the pattern holds, but the precision barely budged. The median trial size in 2008 was 27. In 2025, it was still 27.

L-theanine on its own

L-theanine alone produced less. One outcome survived the statistical filter: choice reaction time at one hour (SMD -0.35, 95% CI -0.61 to -0.10). Everything else (mood, other cognitive measures, any effect at the second hour) failed to separate from placebo.

L-theanine is an amino acid found almost exclusively in Camellia sinensis tea leaves and in one obscure mushroom, Xerocomus badius. It crosses the blood-brain barrier and increases alpha-band activity, the EEG signature of relaxed alertness. People report feeling calmer after taking it. But controlled trials have never found much of a performance boost, and this meta-analysis is no exception. L-theanine may shave a few milliseconds off reaction time. It is not a nootropic.

What about sleep

The title says “sleep.” The evidence base says otherwise. None of the 15 meta-analyzed trials measured sleep in a way that allowed pooling. The authors note this as a gap and call for trials in free-living participants using actual tea beverages, with sleep quality registered as a primary outcome.

What the review leaves open

The trials measured effects across two hours. Nobody knows what happens after that. Whether L-theanine plus caffeine produces a sustained cognitive benefit, or tolerance builds within days, or the effects wash out entirely. All untested.

The participants averaged 32 years old. Older adults metabolize caffeine more slowly and are the demographic most likely to walk into a supplement store for L-theanine capsules. These findings tell them nothing.

Then there is the question nobody in this literature wants to answer directly: does drinking tea produce the same effects as swallowing a capsule. The trials used isolated compounds. A brewed cup involves steep time, water temperature, polyphenols that bind to theanine, and gastric emptying that slows absorption when you drink rather than swallow. It is possible that three or four cups of green tea approximate the doses used in these trials (roughly 100 mg of L-theanine, 50 to 75 mg of caffeine). It is equally possible they do not. No trial has compared the two head-to-head.

The funding deserves a straight look. Two of the five authors work for Lipton and Unilever. The review was prospectively registered on PROSPERO and the authors used Cochrane risk-of-bias tools. The methods appear sound. But the funder sells the product category under investigation, and an independent team running the same search strategy would put the conflict-of-interest question to rest. Until that happens, the findings sit in the “probably real, worth verifying” column.

What this means

A cup of green tea delivers 8 to 30 mg of L-theanine and 30 to 50 mg of caffeine. The doses studied in the meta-analyzed trials were higher: roughly 100 mg of L-theanine and 50 to 75 mg of caffeine, the equivalent of three to four cups consumed at once. The meta-analysis did not test whether a single morning cup moves the needle. It probably does not.

For the person who drinks tea throughout the day, the evidence points toward a mild, real effect on attention that kicks in within an hour or two and lasts until the caffeine wears off. The mechanism is not mysterious: L-theanine promotes alpha-wave activity, caffeine blocks adenosine, and the two together produce a smoother alertness than either alone. Whether this effect matters in practice depends on what you are comparing it to. Next to nothing, it is a genuine lift. Next to coffee, it is weaker and slower.

For anyone buying L-theanine supplements, the distinction matters. A product that combines 100 mg of L-theanine with 50 mg of caffeine has some evidence behind it. A product that contains L-theanine alone and promises sharper focus is hanging its claim on one reaction-time result from trials with 27 people.

References

1. 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. Nutrition Reviews. 2025;83(10):1873-1891. https://doi.org/10.1093/nutrit/nuaf054
2. Haskell CF, Kennedy DO, Milne AL, et al. The effects of L-theanine, caffeine and their combination on cognition and mood. Biological Psychology. 2008;77(2):113-122. https://doi.org/10.1016/j.biopsycho.2007.09.008