Evidence overview
L-Theanine
L-theanine is an amino acid from tea, studied across acute cognition, stress, anxiety, and sleep. The research base is moderate in size, dominated by short-duration trials in healthy adults, and often examines L-theanine in combination with caffeine rather than alone. Recent systematic reviews on sleep outcomes have expanded the literature into newer territory.
The research base is moderate in size, dominated by short-duration trials in healthy adults, and often examines L-theanine in combination with caffeine rather than alone. Recent systematic reviews on sleep outcomes have expanded the literature into newer territory.
Most studied for
Cognitive performance
PubMed coverage
218 studies
Safety profile
Moderate
Mechanism class
Amino acid found primarily in tea leaves. Crosses the blood-brain barrier; modulates glutamate and...
Study coverage
Study coverage by goal
PubMed counts for L-Theanine grouped by the goal each study targets.
Evidence
What the evidence covers
What's been studied. Conclusions live in the full report.
L-theanine is one of the few non-protein amino acids consumed regularly in the diet (via tea) and is a frequent subject of acute-effects research because its onset is fast (under an hour) and its effect profile is easy to measure. The dominant supplemental form in trials is purified L-theanine; combinations with caffeine are the most-studied pairing because the two are co-consumed in tea, and many trials cannot cleanly separate L-theanine effects from caffeine effects.
The outcome dimensions covered most heavily are acute cognitive performance (attention tasks, reaction time, working memory) under stress or fatigue, subjective stress and anxiety markers, mood, and more recently sleep quality. The most-studied populations are healthy young adults, with smaller research streams in clinical anxiety, ADHD, and aging-related cognitive decline. The shorter-than-typical trial durations (single-dose, hours-to-days) reflect the supplement's acute pharmacology rather than a research-quality gap.
Demographically the literature skews toward healthy young adults and university student populations, which is a known limitation when generalizing to older adults, people with diagnosed anxiety disorders, or chronic-stress populations. The newer 2025-2026 sleep syntheses extend the literature into longer trial designs and more diverse demographic samples but are still building toward demographic balance.
What the studies found for the goal you choose is in our full report.
Safety
Safety summary
Common adverse events, drug interactions, and special populations.
L-theanine has an unusually clean safety profile and has a GRAS notice for use in foods that the FDA did not question. Clinical trials have reported no serious adverse events. There is no established tolerable upper limit and dietary intake from tea has been consumed for centuries without identified harm. Drug interactions are limited: theoretical additive effects with antihypertensive medications and CNS depressants have been described but rarely documented in trials at supplement doses. People with low blood pressure or those on stimulant or sedative medications should consult a clinician.
This summary is informational and not medical advice. Consult a clinician before starting or changing any supplement, especially if you take prescription medications.
Our full report covers the doses used in studies, drug interactions and contraindications, and what those studies found for the goal you choose.
Foundations
Foundation of the evidence base
A few studies the field anchors on. Not the full picture, just the starting points.
Recent comprehensive synthesis of L-theanine's effects across cognition, mood, and sleep, both alone and in caffeine combinations. Its breadth makes it a supplement-overall reference for healthy adults; a 2026 meta-analysis of 31 trials extends to clinical populations.
View on PubMedSleep-specific meta-analysis of L-theanine trials, published in the highest-impact sleep-medicine specialty journal. The field's anchor for the newer wave of L-theanine-for-sleep research.
View on PubMedL-theanine-specific systematic review on the stress and anxiety use case, which is the primary marketed and consumer-facing use of the supplement. Establishes the reference document for this specific application.
View on PubMed
Limitations
What this page doesn't answer
Where the public summary stops and the full report begins.
This page summarizes the L-theanine literature at a general level. It does not address what dose is appropriate for your specific goal (acute stress vs. sleep onset vs. chronic anxiety), whether you would benefit more from L-theanine alone or in a combination, how L-theanine interacts with your medications, or whether your baseline tea consumption already provides a meaningful dose. The personalized report works through those questions for the goal you pick.
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Goal-specific evidence
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