Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A…
This 30-person trial was funded by the company that makes the L-theanine and ran for four weeks in people with no diagnosis — how much weight would you put on it for someone in my situation?
Suggestive but preliminary. Not settled. how we score evidence
Caveat on this rating: Funded by Taiyo Kagaku Co., Ltd., which manufactures the Suntheanine-brand L-theanine; two of the seven authors are employees of that company and it supplied both the active and placebo tablets. Several outcomes were measured within-person before-and-after rather than against placebo, which inflates apparent effects.
L-theanine is sold in the US as a dietary supplement under the Dietary Supplement Health and Education Act of 1994 (DSHEA), and as a food additive it holds Generally Recognized as Safe (GRAS) status for use in beverages.
Thirty healthy adults with no major psychiatric illness took 200 mg/day of L-theanine or placebo for four weeks each in a crossover design. Depression, trait anxiety, and sleep quality scores all fell after the L-theanine period (p=0.019, 0.006, and 0.013), and verbal fluency improved. With 30 participants, a four-week exposure, and no diagnosed condition among them, this trial can suggest a direction but cannot establish that L-theanine treats anything.
Worth asking
This 30-person trial was funded by the company that makes the L-theanine and ran for four weeks in people with no diagnosis — how much weight would you put on it for someone in my situation?
Source
Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial. — Hidese S, Ogawa S, Ota M, et al. (2019)
Indexed peer-reviewed journal
Read the source: https://doi.org/10.3390/nu11102362
DOI: 10.3390/nu11102362
How this was scored
- Study design
- Randomised controlled trial
- Funding
- Industry funded
- Published in
- Indexed peer-reviewed journal
- Sample size
- 30
- Preregistered
- No
- Conflicts disclosed
- Yes
- Independent of proponent
- No
- Retracted
- No
Read the full scoring rubric, including what it can't tell you.
Published August 19, 2026.
Questions
How strong is the evidence behind this?
Resolv rates this source "early signal". Suggestive but preliminary. Not settled. It scores 44 out of 100 on our published rubric. One caveat travels with that badge: Funded by Taiyo Kagaku Co., Ltd., which manufactures the Suntheanine-brand L-theanine; two of the seven authors are employees of that company and it supplied both the active and placebo tablets. Several outcomes were measured within-person before-and-after rather than against placebo, which inflates apparent effects. The score is calculated from recorded facts about the source — study design, funding, publication venue, sample size, preregistration — not typed in by an editor.
What is the source for this?
Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial. — Hidese S, Ogawa S, Ota M, et al. (2019). Published in: Indexed peer-reviewed journal. DOI: 10.3390/nu11102362. The full source is linked on this page so you can read it yourself.
Who paid for this research, and does that matter?
Study design: Randomised controlled trial. Funding: Industry funded. The researchers were not independent of whoever benefits from the result. Industry sponsorship is one of the most reliably measured biases in medicine, which is why funding carries real weight in the score rather than sitting in a footnote.
Is l-theanine, theanine, suntheanine, alphawave, gamma-glutamylethylamide, green tea amino acid FDA approved for this use?
No. Its current status is: Not FDA-evaluated for this use. L-theanine is sold in the US as a dietary supplement under the Dietary Supplement Health and Education Act of 1994 (DSHEA), and as a food additive it holds Generally Recognized as Safe (GRAS) status for use in beverages.
Is this medical advice?
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.