This tea-derived amino acid may improve cognition and stress-related symptoms, but studies in psychiatric disorders are preliminary
STUDY: Cotter J and Kilpatrick HE, Frontiers in Nutrition 2026
STUDY TYPE: Narrative mini review of systematic reviews and meta-analyses
FUNDING: Authors employed by manufacturer
Background
L-theanine is an amino acid in tea that structurally resembles glutamate and glutamine, allowing it cross into the brain and modulate neurotransmitters involved in relaxation. As a supplement, it is popular for stress, sleep, and mental focus. This review pooled the placebo-controlled trials behind those claims.
The Study
- Meta-analyses of placebo-controlled trials in healthy adults: cognition (2 meta-analyses, about 150 participants each), mood (a review of 9 trials, 270 participants), and sleep (2 reviews of 13 to 19 trials, 550 to 897 participants).
- Doses across the included trials ranged from 50 to 1,000 mg/day, given alone or combined with caffeine.
Healthy or Stressed Populations
- For mood, 200 to 400 mg/day reduced both perceived and physiological markers of stress and anxiety (salivary cortisol and alpha-amylase) during stressful situations, without causing sedation.
- For sleep, 200 to 450 mg/day improved sleep-onset latency, subjective sleep quality, and next-day functioning, with small-to-medium effect sizes and little effect on total sleep duration.
- For cognition, single doses of 100 to 400 mg produced small improvements on visual attention and reaction-time tasks, especially combined with caffeine, with no effect on simple reaction time.
- L-theanine was generally well-tolerated, with adverse effects similar to placebo.
Psychiatric Populations
- Evidence in people with diagnosed psychiatric conditions is limited. There are a few small placebo-controlled trials in schizophrenia, OCD, depression, anxiety disorders, and ADHD showing benefits.
- In the Carlat CAM Factbook, l-theanine earned a possible rating for augmentation in OCD, schizophrenia, and depression (1-2 small trials each), and a probable rating for cognition in healthy adults (1 large, 9 small trials).
Side Effects
Well-tolerated, low-risk, with adverse effects similar to placebo
Limitations
Both authors are employed by The Water Street Collective Ltd, a parent company that manufacturers L-theanine and other products. Most trials were small, tested single or short-term dosing, and enrolled healthy volunteers rather than patients with diagnosed anxiety, insomnia, or other conditions. Optimal dosing are unclear.
Practice Implications
- In practice, the benefits I’ve seen are mild and easy to miss, especially as when it improves sleep quality without causing sedation.
- With benefits in stress, anxiety, sleep, and cognition, it is a feasible alternative to benzodiazepines; much safer but not as potent.
- A practical starting point is 200 mg once or twice daily with food, reassessed in a couple of weeks. Find lab-tested products here.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
