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L-theanine

Amino acid · Green tea amino acid

L-theanine is an amino acid found almost exclusively in the tea plant. It is considered the substance that cushions the caffeine kick without making you tired. The data behind this are unusually broad for a nootropic — and in one place thinner than the scene assumes.

In short

L-theanine improves attention, and this is well supported. A meta-analysis of 31 randomized trials with 1,168 participants found an improvement in choice reaction time of SMD 0.51 after a single dose of 200 milligrams taken 30 to 60 minutes before the test. The effect on acute stress is smaller (SMD 0.31) and comes mostly from studies with a high risk of bias; no effect on fatigue was detectable. The rule of thumb of combining caffeine and L-theanine in a 1:2 ratio has never been tested against any other ratio. And the alpha waves in the EEG, which are usually used to explain the effect, are a measurement in the brain, not an outcome in everyday life.

What it is

L-theanine is a non-proteinogenic amino acid: the body does not build it into proteins. It occurs in appreciable amounts only in Camellia sinensis, i.e. in green, white and black tea. Shaded Japanese teas such as gyokuro and genuine matcha contain particularly large amounts. Theanine carries the umami taste and dampens bitterness.

In food supplements, L-theanine is almost never present as a tea extract but as an isolated substance produced by a fermentation or enzymatic process. Legally, this makes a difference. The amounts used in the studies are far above what one cup provides: Kahathuduwa and colleagues describe their single dose of 200 milligrams as the equivalent of eight cups of black tea.

How it is supposed to work

The common explanation: L-theanine is structurally similar to glutamate and also acts on the GABA-A receptor. A systematic review describes it as an inhibitor of glutamate reuptake and as a low-affinity glutamate receptor antagonist in the hippocampus.

This chain has not been measured through in humans. What is documented there are only levels further down the line: activity in the EEG, cerebral blood flow, salivary cortisol. The often-quoted claim that L-theanine raises GABA, serotonin and dopamine is based on animal studies — in mice, cortical GABA and serum serotonin were measured; in humans, there are no data on this.

Where the 1:2 ratio comes from

The combination with caffeine is the most frequently mentioned use, and almost always a fixed number comes with it: one portion of caffeine, two portions of L-theanine. Where does it come from?

In 2008, Owen and colleagues compared 50 milligrams of caffeine with and without 100 milligrams of L-theanine in 27 participants. Haskell and colleagues used 250 milligrams of L-theanine and 150 milligrams of caffeine in the same year. Giesbrecht and Einöther worked with 97 milligrams of L-theanine and 40 milligrams of caffeine in 2010. Kahathuduwa and colleagues opted for 200 milligrams of L-theanine and 160 milligrams of caffeine in 2017, as did Nawarathna and colleagues again in 2025. Dodd and colleagues reversed the ratio in 2015 and gave 75 milligrams of caffeine with 50 milligrams of L-theanine, because this is closer to the amounts in tea.

None of these studies tested one ratio against another. Each set exactly one dose pair against placebo. Nor is there any study registered on clinicaltrials.gov that compares mixing ratios. The 1:2 ratio is therefore not a measured optimum but the rounded form of a single early study protocol. That does not mean it is wrong — it means that no one has tested it.

In addition: in 2017, Kahathuduwa did find the largest improvement for the combination, but no statistical interaction between theanine and caffeine — the two added up, they did not potentiate each other. And in 2018, EFSA recognized the attention effect of black tea but attributed it to the caffeine; the claimed interplay of the two substances was not recognized.

Alpha waves are a measurement, not an outcome

Alpha activity in the range of 8 to 14 hertz is considered the signature of an awake, relaxed brain. Nobre and colleagues showed that even 50 milligrams of L-theanine raises alpha activity in resting participants more than placebo. A later study with 200 milligrams found more alpha power frontally and over the entire scalp three hours later, plus a drop in salivary cortisol after one hour.

But: alpha power is a surrogate. It stands for a state, not for a performance — and it does not even behave consistently. Gomez-Ramirez and colleagues found that during a demanding attention task, L-theanine lowered background alpha, while the task-related component increased. In a magnetoencephalography study in 34 adults, resting alpha activity two hours after intake was above placebo, but only in the more anxious participants — and cognitive performance did not change at all.

Sleep and longer-term intake

There is a separate meta-analysis on sleep: 19 studies with 897 participants. It finds small improvements in subjective sleep onset latency (SMD 0.15), daytime sleepiness (SMD 0.33) and overall sleep quality (SMD 0.43) — all questionnaire scores, no measurements in a sleep laboratory.

Over several weeks, the picture is mixed. A crossover study in 30 people found better scores for depressive symptoms (p = 0.019), anxiety (p = 0.006), sleep quality (p = 0.013) and executive function (p = 0.031) after four weeks of 200 milligrams daily. A parallel-group study in 30 moderately stressed adults with 400 milligrams daily over 28 days, by contrast, found a decline in the stress score in both groups. That is the norm with stress scales: they also fall on placebo.

What is well supported

The best-supported effect is the acute effect on attention, and from three independent meta-analyses. The largest finds an improvement in choice reaction time of SMD 0.51 (95 percent confidence interval 0.25 to 0.77) after 200 milligrams, the second SMD -0.35 for L-theanine alone in the first hour, and the third, from five studies with 148 adults, a recognition reaction time that was 15.20 milliseconds shorter.

What the studies show

Meta-analysis of 31 randomized trials

The most comprehensive analysis to date pooled 31 randomized, placebo-controlled trials with a total of 1,168 participants from healthy and clinical groups. The primary endpoint was the acute effect of a single dose on stress in healthy people. A single dose of 200 milligrams, taken 30 to 60 minutes before the test, significantly improved choice reaction time (SMD 0.51). The primary endpoint, stress, was smaller at SMD 0.31 and was driven mostly by studies with a high risk of bias. There was no effect on fatigue, and anxiety remained inconsistent.

Combination with caffeine during sleep deprivation

37 healthy adults aged 22 to 30 stayed awake for one night and, before and 50 minutes after intake, completed a task in which they had to react to traffic scenes with an impending accident and ignore more frequent harmless scenes. 200 milligrams of L-theanine plus 160 milligrams of caffeine were compared with placebo, double-blind in a crossover, accompanied by a 32-channel EEG. The hit rate rose (p = 0.02). Reaction time improved by 52.08 milliseconds on the combination and by 13.97 milliseconds on placebo; the difference of 38.1 milliseconds was significant (p = 0.003).

The study that reversed the ratio

Dodd and colleagues gave 12 habitual and 12 non-habitual caffeine consumers 75 milligrams of caffeine, 50 milligrams of L-theanine, both together and placebo in four sessions, and measured cerebral blood flow, cognition and mood 30 minutes later. Caffeine alone improved attention and mood and constricted the blood vessels. In combination with L-theanine, the vasoconstriction disappeared — but so did the benefit for cognition and mood.

Where the data stop

Three gaps stand out. First, the mixing ratio with caffeine: no study tests one ratio against another. Every statement about an optimum is a stipulation. Second, the chain of action: that L-theanine raises GABA, serotonin or dopamine in humans has not been shown; the measurements come from rodent models. Third, the endpoints: all documented effects are reaction times, test batteries and questionnaire scores, measured over minutes to a few weeks. How daily intake over months plays out is open.

Status, approval and legal

In Germany, L-theanine is not an approved medicine but an ingredient in food supplements. There is no legal maximum amount, and no authorized health claim either: EFSA assessed the applications on cognition, psychological stress, sleep and menstrual complaints in 2011, and theanine does not appear in the list of permitted claims under Regulation (EU) No. 432/2012. The claim on black tea and attention recognized in 2018 is based on the caffeine and requires two to three servings with a combined total of at least 75 milligrams of caffeine within 90 minutes. For isolated L-theanine, a novel food application by Taiyo GmbH from Gevelsberg is pending, because the manufacturing process was not used for food in the Union before May 15, 1997. L-theanine is not on the doping list.

Safety

Tolerability in the studies is consistently good. The meta-analysis of 31 studies compared dropout rates and reasons for dropout between active treatment and placebo and reports no serious adverse events. Still, the substance is not free of side effects: in Haskell and colleagues, headache reports rose on L-theanine alone, and the number of correct subtractions in an arithmetic task fell. The novel food application bases a daily amount of 200 milligrams on sub-chronic and chronic toxicity studies, but restricts use to adults and excludes children and adolescents under 18, pregnant and breastfeeding women. Anyone combining it with caffeine should keep an eye on the amount of caffeine: EFSA and BfR consider up to 200 milligrams as a single dose and up to 400 milligrams over the day safe for healthy adults, and up to 200 milligrams daily for pregnant and breastfeeding women.

BK-Score Supported, with caveats

Human evidence7
Mechanism5
Safety data6
Hype gap5
Track record of use6

An unusually broad data base for a nootropic: a meta-analysis of 31 RCTs with 1,168 participants (Molecular Psychiatry 2026) finds an improvement in choice reaction time of SMD 0.51 after a single dose of 200 milligrams, and two further meta-analyses (Nutrition Reviews 2025, Journal of Clinical Medicine 2025) point in the same direction. The stress effect (SMD 0.31) is driven mostly by studies with a high risk of bias, and anxiety and fatigue remain inconsistent. The mixing ratio with caffeine has never been tested against any other: every study set exactly one dose pair against placebo, and in 2018 EFSA attributed the attention effect of black tea to the caffeine alone. The GABA narrative comes from animal data, and the alpha waves in the EEG are a surrogate. The effects are real, but smaller and measured over shorter periods than advertised.

The score rates the state of knowledge, not the substance. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about L-theanine

How quickly does L-theanine work?

In the studies, the single dose was usually taken 30 to 60 minutes before the measurement. In the sleep deprivation study, the measurement window was 50 minutes after intake. For alpha activity in the EEG, by contrast, the clearest difference was found only three hours after intake. So there is no uniform time window.

Is L-theanine from tea the same as from a capsule?

Chemically yes, in quantity no. According to the authors, the 200 milligrams used in studies correspond to about eight cups of black tea. Legally, isolated L-theanine is also the subject of a pending novel food application, while tea has always been allowed on the market as a food.

Does L-theanine make you tired?

In the meta-analysis of 31 studies, no significant effect on fatigue was detectable. In this respect, the description as relaxing without making you tired fits the data. With evening intake, sleep studies showed small improvements in subjective sleep onset latency, but exclusively in questionnaire scores.

Does L-theanine cancel out the effect of coffee?

That depends on the amount. With 75 milligrams of caffeine and 50 milligrams of L-theanine, the benefits of caffeine for cognition and mood disappeared completely in one study. At higher amounts such as 160 milligrams of caffeine and 200 milligrams of L-theanine, on the other hand, the combination showed the strongest effects. The question has not been settled.

What do the alpha waves mean?

Alpha activity between 8 and 14 hertz is considered a sign of an awake, relaxed brain. It is a surrogate marker, i.e. a measurement that stands for a state, not for a performance. During a demanding attention task, L-theanine even lowered background alpha instead of raising it.

Who is L-theanine not intended for?

The pending European novel food application provides for use by adults only and excludes children and adolescents under 18, pregnant and breastfeeding women. There are hardly any robust data on interactions with medications. If you have an existing condition or take medication, the question belongs in the hands of a physician.

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Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in the hands of a physician. Last updated: 2026-10-04.