L-Theanine Dosage: What the Studies Used
Take 200 mg once: the dose most one-dose trials used. Health Canada’s monograph lists 200 to 250 mg a day for adults; no U.S. agency sets a dose. Daily trials in healthy adults used 200 to 400 mg a day. With caffeine, trials paired about 0.8:1 to 2.4:1 L-theanine to caffeine. Take the serving on your label, and check whether it means 1 capsule or 2.
By Erin Rose Updated October 7, 2026 How we score
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How much, by situation
Find your situation. These amounts are from trials in healthy adults.
| Your situation | How much | Notes |
|---|---|---|
| One dose, as most trials used | 200 mg | One-dose trials used 50 to 400 mg; 200 mg was the most common. More |
| With caffeine | About 0.8:1 to 2.4:1 | 97 to 250 mg of L-theanine with 40 to 250 mg of caffeine. No trial compared one ratio with another. More |
| Daily, for weeks | 200 to 400 mg a day | Both daily trials in healthy adults ran about 4 weeks (28 days): one at 200 mg a day, one at 400 mg. More |
| Health Canada’s monograph | 200 to 250 mg a day | For adults 18 and older. It is Canada’s guide for licensing L-theanine products. No U.S. agency sets a dose. More |
| More than 400 mg at once | Not tested | No trial we read tested more than 400 mg. Up to 400 mg, results were mixed: 400 mg did not speed reaction time (2023) but was the only dose that changed a brain-signal measure (2022). More |
What doses the trials used
One-dose trials used 50 to 400 mg. 200 mg was the dose used most; a 2026 review of 31 trials found one 200 mg dose, taken 30 to 60 minutes before testing, sped a choice reaction-time test (Gerolymos 2026; one author founded a supplement company).
| Item | Value |
|---|---|
| Nobre 2008 | 50 mg |
| Lu 2004 | 200 mg |
| Dassanayake 2023 | 100 to 400 mg |
| Dassanayake 2022 | 100 to 400 mg |
| Owen 2008 | 100 mg |
| Giesbrecht 2010 | 97 mg |
| Haskell 2008 | 250 mg |
| Kahathuduwa 2017 | 200 mg |
| Rogers 2008 | 200 mg |
| Trial | Daily dose | What it measured |
|---|---|---|
| Hidese 201930 healthy adults (crossover) | 200 mg a day for 4 weeks | Mood, stress and sleep-quality questionnairesTies: Taiyo Kagaku (maker of Suntheanine) supplied the tablets; two authors work there |
| Moulin 202430 healthy adults with moderate stress | 400 mg a day for 28 days | A stress questionnaire and saliva cortisol, at day 14 and day 28Ties: Run by a contract research firm, with an author from the maker of AlphaWave, the L-theanine tested |
What each trial found, with its funder: the research, read straight.
The trials that compared doses
In 32 young adults, one dose of 100, 200 or 400 mg was tested against a dummy pill. 100 and 200 mg sped a simple reaction-time test by about 16 thousandths of a second; 400 mg did not (Dassanayake 2023). In a sister trial, only 400 mg changed a brain-signal timing measure (Dassanayake 2022). So more was not clearly better.
Doses with caffeine
| Study | L-theanine + caffeine | Ratio |
|---|---|---|
| Owen 2008First author at Unilever (PubMed address); no funder named | 100 mg + 50 mg | 2:1 |
| Giesbrecht 2010First author at Unilever (PubMed address); no funder named | 97 mg + 40 mg | 2.42:1 |
| Haskell 2008None named on PubMed | 250 mg + 150 mg | 1.67:1 |
| Kahathuduwa 2017None named on PubMed | 200 mg + 160 mg | 1.25:1 |
| Rogers 2008None named on PubMed | 200 mg + 250 mg | 0.8:1 |
The pairs ran from about 0.8:1 to 2.4:1, not one fixed ratio. 200 mg with 100 mg of caffeine (2:1) sits inside that range. In Rogers 2008, L-theanine did not change jitteriness, though it blunted the rise in blood pressure from 250 mg of caffeine.
How many capsules make 200 mg
Capsule and chewable labels we score hold 100 mg, 150 mg or 200 mg a piece. 4 of them need 2 pieces for 200 mg; one 150 mg label needs 2 for 300 mg. And a “400 mg” label can mean 2 capsules of 200 mg. Check the serving size on the back:
| Per piece | For 200 mg | Products |
|---|---|---|
| 200 mg | 1 piece | 12 products |
| 150 mg | 2 pieces (300 mg) | 1 product |
| 100 mg | 2 pieces | 4 products |
When to take it
In the one-dose trials we read, testing came 20 to 90 minutes after the dose. The 2026 review’s 200 mg result was for doses taken 30 to 60 minutes before testing (Gerolymos 2026). No trial we read compared times of day or taking it with and without food.
Side effects and cautions
We found no upper limit set by a national health agency. In one trial, 250 mg alone raised “headache” ratings (Haskell 2008). If you take any medicine, are pregnant, or are buying it for a child, ask your doctor first. More on side effects.
Questions
How much L-theanine should I take?
No U.S. agency sets a dose. Health Canada's L-theanine monograph lists 200 to 250 mg a day for adults. Most one-dose trials used 200 mg, and the range was 50 to 400 mg. Start with the serving on your label, and check whether it is 1 or 2 capsules.
What ratio with caffeine?
The trials used about 0.8:1 to 2.4:1 L-theanine to caffeine, such as 100 mg with 50 mg of caffeine or 200 mg with 160 mg. 200 mg with 100 mg (2:1) is inside that range. None compared one ratio with another.
Is more than 400 mg better?
No trial we read tested more than 400 mg at once. In a trial that compared 100, 200 and 400 mg, 100 and 200 mg sped a simple reaction-time test and 400 mg did not (Dassanayake 2023). In a sister trial, only 400 mg changed a brain-signal measure (Dassanayake 2022).
Sources
- Nobre AC 2008, Asia Pacific journal of clinical nutrition. PubMed 18296328
- Kimura K 2007, Biological psychology. PubMed 16930802
- Lu K 2004, Human psychopharmacology. PubMed 15378679
- Dassanayake TL 2023, Nutritional neuroscience. PubMed 36263942
- Dassanayake TL 2022, Nutritional neuroscience. PubMed 32777998
- Owen GN 2008, Nutritional neuroscience. PubMed 18681988
- Giesbrecht T 2010, Nutritional neuroscience. PubMed 21040626
- Haskell CF 2008, Biological psychology. PubMed 18006208
- Kahathuduwa CN 2017, Nutritional neuroscience. PubMed 26869148
- Rogers PJ 2008, Psychopharmacology. PubMed 17891480
- Hidese S 2019, Nutrients. PubMed 31623400
- Moulin M 2024, Neurology and therapy. PubMed 38758503
- Gerolymos C 2026, Molecular psychiatry. PubMed 42410082
- Camfield DA 2014, Nutrition reviews. PubMed 24946991
- Health Canada, Natural Health Product Monograph: L-Theanine (dated January 30, 2026), read October 7, 2026. Monograph
- Labels: each brand’s page or the NIH Dietary Supplement Label Database, read October 7, 2026.
This page is for information. It is not medical advice.