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Open WellbeingWhat the studies actually measured

Mood, stress and thinking

L-theanineand Cognitive performance

Only one source here so far, and it measured higher cognitive performance.

How sure anyone can be

Low certainty, level 2 of 4

Further research is likely to change this.

The numbers reported

One source’s range, drawn against its own line of no difference and scaled to its own numbers.

0.250.51 standardised effect size0.77

This range stays on one side of the no-difference line.

Bar the confidence interval — the range the true effect is likely to sit inNotch the number the source reportedLine where there would be no difference at all

One source, newest first

What we read is the published abstract, not the full paper, so a blank below may still be printed in the paper itself. A fixed rule worked this out from the source’s own fields. Nobody checked this rating by hand.

  • Systematic review, 2026

    Low certainty, level 2 of 4

    Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials.

    Mol Psychiatry. 2026. doi:10.1038/s41380-026-03727-9. PMID:42410082.

    Number reported
    0.51 standardised effect size
    Which way it went
    Higher
    Likely range
    0.25 to 0.77
    Source’s own words
    standardised mean difference in choice reaction time
    Who was studied
    Adult
    Compared with
    placebo
    Amount taken
    200 mg, single dose
    For how long
    0 days
    Disagreement between studies
    Not in the abstract
    Kind of record
    Journal article
    How this rating was reachedstarts at High, 2 steps down: risk of bias and imprecision
    1. Starts at High
    2. Risk of bias brought it down to Moderatethe abstract's own appraisal reports serious study limitationsReported in the abstract
    3. Imprecision brought it down to Lowthe abstract does not state how many participants were pooledNot in the abstract
    4. Ends at Low2 steps down.
    Checked, nothing to step it down for
    indirectness
    Inconsistency
    No disagreement reported, and no count of studies behind it

    Imprecision stops at the first check that applies; others may apply too.

    AI review could not confirm it — an AI agent re-read this finding against the saved source and at least one check failed. The batch record names the check and quotes the sentence that caused the doubt. No person read it.

    Read the abstract on PubMed

What is still missing from these readings

  • One do not say how many people took part.

Something nobody wrote down is never a zero and never a guess.

Still to read for L-theanine

Five of the 12 things on our list for L-theanine have no reading here yet:

  • Alpha EEG power
  • Diastolic blood pressure
  • Heart rate variability (RMSSD)
  • Resting heart rate
  • Systolic blood pressure

Still to read means nobody here has got to it, not that it was tried and did nothing or that no research exists. A pairing on no list is not a gap: we never set out to read it.

Read next

This is a library, not advice. No recommendation, no dose, no diagnosis, no ranking. We never ask about your symptoms, and nothing here is tailored to you.