supplementfocus

L-Theanine (alone, without caffeine)

Best evidence: C — Preliminary / mixed for Attention & reaction time

A calming amino acid that shows no clear attention benefit on its own in recent meta-analysis — it mainly helps when paired with caffeine.

Graded outcomes

What the evidence says

Preliminary / mixed · Suggestive but unsettled — could go either way.

Effect size: No clear effect on reaction time or selective attention when taken alone, per the most recent meta-analysis; effects emerge only in combination with caffeine (small-to-moderate, e.g. attention-switching accuracy SMD = 0.33).
Evidence base: A 2025 meta-analysis pooled 5 RCTs for the alone-vs-placebo comparison in healthy adults; the theanine+caffeine literature is somewhat larger.
Population: Findings are specific to healthy adults without cognitive impairment; some signal of benefit in older adults with mild cognitive impairment appears in secondary/subgroup analyses, not yet confirmed.
Dosage / protocol: 200 mg as a single dose is the most commonly studied amount in acute trials.
Contraindications & cautions: Very well-tolerated; minimal known interactions.

Citations

Compared with