L-theanine

This page is a reference summary of published evidence and regulatory documents. It is not a substitute for individual medical advice, and it is not a recommendation to take or avoid any medicine.

Medically reviewed by Dr. Sofia Lindqvist, · Last reviewed 12 February 2026

L-theanine is an amino acid found naturally in the leaves of Camellia sinensis, the tea plant, and sold as a supplement in doses typically between 100mg and 400mg. This page is educational: it describes what has been measured and what regulators have said, and is not a comparison against any prescription medicine.

What it is

L-theanine (N-ethyl-L-glutamine) occurs naturally in tea leaves and in small amounts in some mushrooms. Supplement products typically isolate or synthesise it and sell it in capsules ranging from 100mg to 400mg per serving, often marketed for relaxation or as a companion ingredient alongside caffeine in beverages and supplements.

The evidence, with effect sizes

A pooled analysis across 31 controlled trials found a standardised mean difference (SMD) of 0.31 for reduction in acute, situational stress measures, an effect size generally described as small-to-moderate. This is the most commonly cited quantitative summary in the L-theanine literature.

Findings on attention and alertness are less consistent when L-theanine is tested alone; the more replicated attention results come from studies combining it with a stimulant ingredient, where the combination produced measurable changes in sustained-attention task performance greater than either ingredient administered alone at the tested doses.

Pooled effect on acute stress across 31 trials: SMD 0.31 — small-to-moderate. Standalone attention and alertness data are inconsistent; combination-ingredient studies show larger, more replicated effects than single-ingredient studies.

Regulatory status

The European Food Safety Authority (EFSA) reviewed proposed health claims relating to relaxation and to maintaining attention, and in its 2011 opinion concluded that a cause-and-effect relationship had not been established between consumption of the ingredient and either claimed effect, based on the evidence submitted at the time. No such claim is currently authorised in the EU. It is sold as a food supplement, not as a medicine, in most markets.

Safety data

Across the available trials, reported adverse effects at typical supplement doses have been mild and infrequent — occasional headache or gastrointestinal discomfort — with no serious adverse events attributed to L-theanine at the doses studied. Long-term safety data beyond the duration of typical trials (days to a few weeks) is limited.

Frequently asked questions

What is L-theanine?

An amino acid found naturally in Camellia sinensis (tea) leaves, sold as a supplement in doses typically between 100mg and 400mg.

How large is the evidence for L-theanine and stress?

A pooled analysis across 31 trials found a standardised mean difference of 0.31 for acute stress reduction, described as small-to-moderate.

Has L-theanine been approved for any health claim?

Reported adverse effects in trials are mild and infrequent, mostly headache or gastrointestinal discomfort, at typical supplement doses.

Is L-theanine safe?

EFSA considers single doses up to 200mg and total daily intake up to 400mg unlikely to raise safety concerns for most healthy adults.