amino acid
L-Theanine: What It Is, Tea Sources and Safety
Also known as theanine, L-theanine supplement, green tea amino acid, L-theanine and caffeine
Short answer
L-theanine is an amino acid found mainly in tea leaves; a 200 ml cup of tea has 6 to 32 mg [1]. It is sold as a supplement, often with caffeine. Studies on relaxation, attention and sleep are small and inconsistent [1][2]. No US or UK agency sets a daily amount, and safety data are mostly from animal studies [1].
Last reviewed · 4 sources
What is L-Theanine?
L-theanine is a non-protein amino acid from the tea plant (Camellia sinensis) that is structurally similar to glutamate and glutamine but is not used to build proteins [1]. It is well absorbed from the gut and crosses the blood-brain barrier [1]. It gives green tea its umami taste [1]. Research is studying it for relaxation, attention and sleep, but reviewers call the evidence limited and inconsistent [1][2].
Forms of L-Theanine you'll see on labels
L-theanine supplements
Available in synthetic form and marketed for relaxation and focus [1]. Supplement doses (50–250 mg in studies) are higher than a cup of tea [1].
L-theanine with caffeine
A common pairing; studies test combinations such as 100 mg L-theanine with 50 mg caffeine [1][3].
Tea
White, green, oolong and black teas contain 5.8 to 32 mg per 200 ml cup; content varies with cultivar, season and brewing [1].
How L-Theanine appears on a label
L-theanine has no FDA Daily Value, so it appears on the Supplement Facts panel by weight (mg) with a symbol pointing to "Daily Value not established" [4]. If it is part of a proprietary blend, only the total weight of the blend has to be shown [4].
When to take L-Theanine
No source we checked names a best time of day or says whether to take it with food. In human studies, blood levels peaked about 0.8 hours after 100 mg, and brain alpha-wave changes were seen 30 to 60 minutes after a dose [1].
What to keep apart from L-Theanine
Caffeine (coffee, tea, energy products)
Studied together for alertness and attention; a 2025 meta-analysis found small-to-moderate differences in the first two hours but wide uncertainty [3].
Green tea
Tea already contains L-theanine; one lab study found green tea may reduce how much L-theanine is absorbed [1].
Prescription medicines
The reviews we used do not report drug interaction studies. Ask your pharmacist before combining.
Foods that contain L-Theanine
- Black, green, oolong and white tea
- Xerocomus badius (bay bolete) mushroom
Who should ask a doctor or pharmacist first
No NIH, FDA or NHS fact sheet covers L-theanine, and the reviews we used do not cover pregnancy, breastfeeding or children. A 2025 review urges caution with supplement-level doses in the wider population until better human trials exist [1]. Ask a doctor or pharmacist first if you are pregnant or breastfeeding, want to give it to a child, or take any medicine.
Keep L-Theanine in a routine that fits
Supin schedules what you take by time of day, reminds you, and tells you when a bottle runs low.
L-Theanine: questions people ask
Studies show blood levels peak about 0.8 hours after a 100 mg dose, with brain alpha-wave changes 30 to 60 minutes after intake [1].
A review found that doses used for relaxation did not have hypnotic or sedative effects [1].
Results are inconclusive. One network meta-analysis found L-theanine did not outperform placebo for anxiety disorders [1].
No. L-theanine is one amino acid in tea, while tea and green tea extracts also contain catechins such as EGCG [1].
Sources
- l-theanine: From tea leaf to trending supplement – does the science match the hype for brain health and relaxation? (Dashwood R, Visioli F. Nutrition Research, 2025) — Nutrition Research (peer-reviewed review, via PubMed)
- The effects of L-theanine consumption on sleep outcomes: A systematic review and meta-analysis (Bulman A et al. Sleep Medicine Reviews, 2025) — Sleep Medicine Reviews (peer-reviewed, via PubMed)
- Effects of Tea 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 (Payne ER et al. Nutrition Reviews, 2025) — Nutrition Reviews (peer-reviewed, via PubMed)
- 21 CFR 101.36 – Nutrition labeling of dietary supplements — U.S. Food and Drug Administration (Electronic Code of Federal Regulations)
Educational information, not medical advice. The amounts above are official reference values, not a recommendation for you. Check with your doctor or pharmacist before starting or changing supplements, especially if you take medication, are pregnant or have a health condition. Supin sells a subscription, not supplements. Last reviewed .