L-Theanine
L-theanine is an amino acid found in green and black tea. Small studies suggest it may support a relaxed but alert state, but the evidence is limited. It's often paired with caffeine, but one study found it didn't reduce caffeine jitters.
L-theanine is an amino acid found in green and black tea, and in some mushrooms. It's associated with a relaxed but alert mental state. It's often taken with caffeine; one trial found the pair improved accuracy on a task-switching test, but a separate trial found theanine didn't reduce caffeine jitters.
Research & evidence
Strong support Some support Weak or no supportColours show how well the sources back each point. The reason is next to each one.
L-theanine increases alpha brain wave activity, a pattern associated with a calm, relaxed but alert mental state — an effect first reported in 1999 and replicated at a lower dose (50mg) in 2008. A more recent (2025) attempt to replicate this in middle-aged adults found the same directional trend but did not reach statistical significance. Note: the first (1999) finding was published by researchers connected to the company that was already supplying L-theanine as "Suntheanine" at the time. The 2008 replication was co-authored by an employee of Unilever, which "markets food products some of which contain L-theanine and caffeine."
Limited evidenceSome support. Results are mixed
Study details
Nobre et al. 2008 (n=16 L-theanine vs. n=19 placebo) gave 50mg (the paper notes a cup of black tea contains about 20mg) and found increased alpha activity (p<0.05), extending Juneja et al. 1999's earlier 50-200mg finding. One of the 2008 authors is a Unilever employee; the paper discloses that Unilever "markets food products some of which contain L-theanine and caffeine." The 1999 paper reads as promotional material for a branded ingredient: "we are now able to supply Suntheanine (trade name of L-theanine)"; "Taiyo Kagaku Co., Ltd, Japan won the 1998 'Food Ingredient Research Award' for development of Suntheanine." A 2025 replication in middle-aged adults (n=30) found the same direction (higher alpha power with L-theanine), but the difference was not statistically significant.
Source: Nobre AC, Rao A, Owen GN. "L-theanine, a natural constituent in tea, and its effect on mental state." Asia Pac J Clin Nutr. 2008;17 Suppl 1:167-8; Juneja LR, Chu DC, Okubo T, Nagato Y, Yokogoshi H. "L-theanine — a unique amino acid of green tea and its relaxation effect in humans." Trends Food Sci Technol. 1999;10(6-7):199-204; Yern CI, Saengmanee N, Sittiprapaporn P. "Effects of Oral Supplement L-Theanine on Relaxation Indexed by Alpha Brainwave." Greater Mekong Subregion Medical Journal. 2025;5(2):101. View source
Combining L-theanine with caffeine has been shown to improve accuracy on task-switching and self-reported alertness in at least one trial, though not other measures of attention. A separate trial found theanine did not significantly reduce caffeine-induced jitteriness or anxiety — it only blunted caffeine's effect on blood pressure.
Limited evidenceSome support. Based on one or a few small trials
Study details
Giesbrecht et al. 2010 (n=44, 97mg theanine + 40mg caffeine): "significantly improved accuracy during task switching and self-reported alertness... reduced self-reported tiredness. There were no significant effects on other cognitive tasks, such as visual search, choice reaction times, or mental rotation." Lead authors Giesbrecht and Rycroft are Unilever-affiliated. Rogers et al. 2008 (n=48, 200mg theanine + 250mg caffeine, a different dose ratio): "Theanine antagonised the effect of caffeine on blood pressure but did not significantly affect jitteriness, alertness or other aspects of mood." That study was run at the University of Bristol, but its Acknowledgments state that Unilever donated the theanine-containing drinks.
Source: Giesbrecht T, Rycroft JA, Rowson MJ, De Bruin EA. "The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness." Nutr Neurosci. 2010;13(6):283-290; Rogers PJ, Smith JE, Heatherley SV, Pleydell-Pearce CW. "Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together." Psychopharmacology. 2008;195(4):569-577. View source
L-theanine modestly reduces acute physiological stress markers (salivary cortisol) in single-dose studies. A separate chronic (4-week) dosing trial found no effect on cortisol or immune markers specifically, though it did find improved self-reported depression, anxiety, and sleep scores over the same period.
Mixed evidenceSome support. Results are mixed
Study details
White et al. 2016 (n=34, single 200mg dose, crossover): "salivary cortisol response to the stressor was reduced three hours post-dose." It was funded by a grant from Neurobrands (CA, USA), a beverage company; the paper states the funders had no input into the design, conduct or interpretation of the study. The L-theanine in the drink was supplied by Blue California. Hidese et al. 2019 (n=30, mean age 48.3, 200mg/day for 4 weeks) found "no significant effect on cortisol or immunoglobulin A levels," but the same study found "the Self-rating Depression Scale, State-Trait Anxiety Inventory-trait, and Pittsburgh Sleep Quality Index scores decreased after L-theanine administration (p=0.019, 0.006, and 0.013)." Two authors (Yasukawa, Ozeki) are employees of Taiyo Kagaku Co., Ltd. (the Suntheanine maker), which also funded the study and supplied the L-theanine.
Source: White DJ, de Klerk S, Woods W, Gondalia S, Noonan C, Scholey AB. "Anti-Stress, Behavioural and Magnetoencephalography Effects of an l-Theanine-Based Nutrient Drink." Nutrients. 2016;8(1):53; Hidese S, Ogawa S, Ota M, Ishida I, Yasukawa Z, Ozeki M, Kunugi H. "Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial." Nutrients. 2019;11(10):2362. View source
Dosage
| Form | Amount |
|---|---|
| Capsule | There's no official recommended dose. The studies on this page used single doses of 50–200 mg, and 200 mg a day for 4 weeks. |
| Tea | Green and black tea contain L-theanine, but supplements are stronger than what you get from tea. |
Side effects
Well documented Reported (case reports, anecdotal or one study) None reportedThe circle shows how well documented an effect is, not how serious it is. A rarely reported effect can still be serious, so always read ‘When to stop’.
| Effect | Frequency | When to stop |
|---|---|---|
| No side effects reportedNone reportedNo side effects reported by our sources | None reported | Stop and talk to a doctor if anything unusual happens. |
Safety & interactions
Safety
No side effects have been reported from L-theanine itself, but data in humans are limited, and supplements are stronger than the L-theanine you get from tea.
Contraindications
Don't take L-theanine if you're allergic to green tea, if you're pregnant or breastfeeding (it may not be safe), or if you take sedatives — including sleep medications such as lorazepam, diazepam or zolpidem — since together they may increase drowsiness.
Drug interactions
L-theanine may add to drowsiness from sedative medications. MSK lists sedatives as a contraindication, and its patient guidance names lorazepam, diazepam, and zolpidem. Its only interaction data is an animal study showing additive or synergistic effects with midazolam, with clinical relevance in humans not yet established.
Established: stated plainly by an authoritative source. Documented: supported by case reports or trial data. Theoretical: based on mechanism, animal data, or a cautious listing. Always check with your pharmacist or prescriber.