Amino Acid
L-Theanine
Strong EvidenceWell-replicated for acute relaxed alertness (alpha waves) and the caffeine-combination attention effect; a 4-week RCT also supports stress, sleep and executive function. The strongest data are with caffeine: 250 mg theanine plus 150 mg caffeine gave faster reaction times, better sentence verification and lower 'tired' ratings than placebo in a crossover trial (Haskell 2008), and 100 mg plus 50 mg improved attention-switching in 27 volunteers (Owen 2008). On its own, 200 mg a day for four weeks in 30 adults lowered anxiety, depression and sleep-quality scores and improved verbal fluency (Hidese 2019, tablets supplied by the theanine maker), and a systematic review of nine controlled studies supports 200–400 mg a day for people under stressful conditions while asking for larger, longer trials (Williams 2020). The limit is clinical anxiety: in 46 adults with diagnosed generalised anxiety disorder, 450–900 mg a day added to antidepressants for eight weeks did no better than placebo on anxiety or insomnia severity (Sarris 2019). Compared with apigenin, the other evening calming entry here, theanine has far more human trial data; compared with ashwagandha its effect is acute, within the hour, rather than built over weeks. Verdict: a low-risk, well-evidenced tool for calm focus on the day, especially alongside coffee — not a treatment for an anxiety disorder.
Recommended Dose
100–200 mg daily
Timing
Morning with caffeine, or evening for relaxation
Mechanism of Action
L-theanine is an amino acid found almost only in tea leaves; a cup of black tea supplies roughly 20 mg (Nobre 2008). Given by mouth to rats it reaches the brain through the blood-brain barrier via a leucine-preferring transporter and raises serotonin and dopamine in several brain regions (Yokogoshi 1998). It is described as blocking glutamate's binding to its receptors, damping cortical excitation (Kimura 2007), and in people EEG recordings show more alpha-band activity — the relaxed-but-awake signature — even at 50 mg, a dose close to a strong cup of tea (Nobre 2008).
Side Effects & Safety
Very well-tolerated. Extremely rare reports of mild headache or dizziness. No known serious side effects. In a six-week trial, 400 mg a day produced no significant adverse events in 98 boys aged 8–12 (Lyon 2011); taken alone in one crossover, 250 mg slightly raised 'headache' ratings and lowered serial-subtraction scores (Haskell 2008).
Do not use / talk to a doctor first
- No absolute contraindications at typical doses
- Pregnancy/breastfeeding — limited data, consult a clinician
Interactions
Blood-pressure (antihypertensive) medication
Possible additive blood-pressure lowering — monitor
Source: Yoto 2012 (PMC3518171)
Caffeine
Favourable pairing — offsets jitter and blood-pressure rise
Source: Haskell 2008 (PMID 18006208)
Prescription sedatives
Mild additive relaxation
Source: Mechanistic (GABA/glutamate modulation)
Pregnancy & breastfeeding
Limited human safety data during pregnancy and breastfeeding; keep to tea-level intake and consult a clinician before supplementing.
Who should avoid / see a doctor
Very low risk overall. Monitor if on blood-pressure medication; consult a clinician if pregnant/breastfeeding or combining with prescription sedatives. Not a treatment for a diagnosed anxiety or sleep disorder.
Synergies
Used in These Stacks
See L-Theanine inside a full protocol — the stack pages cover when and why to combine it.
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References
- 1.Nobre 2008, Asia Pac J Clin Nutr — Increased alpha-wave activity / relaxed alertness. (short supplement article — the alpha-wave finding is the replicated core)
- 2.Haskell 2008, Biol Psychol — Placebo-controlled crossover: 250 mg L-theanine + 150 mg caffeine gave faster simple and numeric working-memory reaction times, better RVIP and sentence-verification accuracy, and lower 'tired' and 'headache' ratings; L-theanine alone raised 'headache' ratings and lowered correct serial sevens.
- 3.Hidese 2019, Nutrients — 200 mg/day for 4 weeks (crossover, n=30) reduced anxiety, depression and PSQI scores and improved verbal fluency and executive function. (two co-authors employed by Taiyo Kagaku, which supplied the tablets)
- 4.Yoto 2012, J Physiol Anthropol — 200 mg attenuated the stress-induced blood-pressure rise in high-stress responders — basis for the blood-pressure-medication caution.
- 5.Owen 2008, Nutr Neurosci — 27 healthy volunteers: 100 mg L-theanine + 50 mg caffeine improved speed and accuracy of attention-switching at 60 min and reduced susceptibility to distraction at 60 and 90 min versus placebo.
- 6.Williams 2020, Plant Foods Hum Nutr — Systematic review of 9 controlled studies of pure L-theanine supplements: 200–400 mg/day may help reduce stress and anxiety in people exposed to stressful conditions; longer and larger trials needed. (systematic review)
- 7.Sarris 2019, J Psychiatr Res — 46 adults with DSM-5 generalised anxiety disorder on stable antidepressants, 8-week double-blind RCT: adjunctive L-theanine 450–900 mg/day did not outperform placebo on HAM-A anxiety (p=0.73) or Insomnia Severity Index (p=0.35); greater self-reported sleep satisfaction (p=0.015). (negative primary outcome in a clinical population)
- 8.Yokogoshi 1998, Neurochem Res — Rats: orally administered theanine entered the brain via the leucine-preferring transport system and increased serotonin and/or dopamine concentrations, especially in striatum, hypothalamus and hippocampus. (animal study)
- 9.Kimura 2007, Biol Psychol — 12 participants, double-blind counterbalanced: L-theanine reduced heart-rate and salivary IgA responses to a mental-arithmetic stressor versus placebo.
- 10.Lyon 2011, Altern Med Rev — 98 boys aged 8–12 with ADHD, randomised double-blind: 400 mg/day L-theanine for 6 weeks raised actigraphy sleep percentage and sleep efficiency; sleep latency unchanged; no significant adverse events.
