L-theanine is an amino acid found almost only in tea leaves. Free Will, a dietary supplement, carries a labeled 250 mg per scoop, paired with 150 mg of caffeine: the two doses tested together in a placebo-controlled crossover study of attention, reaction time and mood in 24 healthy adults. 8
Across small trials, theanine plus caffeine has shown small-to-moderate improvements over placebo in attention-switching accuracy and alertness, measured in the first two hours after a dose. 12,17 Evidence for theanine on its own, for stress, is early and mixed, and theanine has not been shown to remove caffeine's "jittery" feeling. 13,15
Key facts
| Fact | Detail |
|---|---|
| Names | L-theanine; γ-glutamylethylamide; N-ethyl-L-glutamine. Named "theanine" by the chemist who first described it in tea. 1 |
| Where it comes from | The tea plant, Camellia sinensis. It is the amino acid behind much of tea's savory (umami) taste. 18 Shaded Japanese teas such as gyokuro and true matcha are often particularly rich in it; the 1950 report measured about 1% of gyokuro leaf. 1,3 |
| In Free Will | A labeled 250 mg of L-theanine per scoop (see the Supplement Facts panel). The source and manufacturer of our L-theanine will be published here once the first production batch and its paperwork are in hand. |
| Doses used in research | 50–250 mg single doses in the attention and EEG studies; 6,8 200 mg per day for four weeks in one crossover trial; 14 200–400 mg per day in the stress review. 15 |
| How fast it absorbs | Blood levels peaked about 50 minutes (0.8 h) after a 100 mg dose in 12 people, whether taken as a capsule or as green tea. 4 |
| Pairs with | Caffeine, 150 mg per scoop. The 250/150 pairing is the one tested in the Haskell 2008 trial. 8 |
| Regulatory status | In 2007 FDA responded "no questions" to a GRAS notice for one manufacturer's L-theanine at up to 250 mg per serving in specified foods. That response covers that notified ingredient and those uses; it is not an approval of any product. 16 |
Why it's in Free Will
Free Will is built around a moderate caffeine dose, and L-theanine is the ingredient we pair with it. We chose 250 mg of theanine with 150 mg of caffeine because those are the doses in the Haskell 2008 trial: a randomized, double-blind, placebo-controlled crossover in which 24 healthy adults (9 men, 15 women, mean age 21) each drank theanine alone, caffeine alone, both, and placebo in a flavored drink on separate days, with testing 30 and 90 minutes after each dose. 8
Compared with placebo, the combination produced faster simple reaction time, faster numeric working-memory reaction time, better sentence-verification accuracy and better rapid-visual-information-processing accuracy, along with lower "tired" and "headache" ratings and higher "alert" ratings. Caffeine alone also beat placebo on several measures. The trial compared each active drink with placebo; it was not designed to show that the combination beats caffeine alone, and we don't claim that. 8
What the evidence does not show, and what we don't claim: that the pairing removes caffeine's side effects, that this ratio is optimal, that the effects last all day, or that the whole Free Will formula behaves the same way. The results come from one lab session per condition, in a small group, measured within 90 minutes. A 2014 review of 11 acute studies of tea constituents found moderate effect sizes for theanine plus caffeine on alertness and attention-switching accuracy in the first two hours after dosing; an exploratory analysis suggested the effect size related more to the caffeine dose than to the theanine dose. 12 A 2025 meta-analysis of trials in healthy people reached a similar, cautious conclusion: small-to-moderate effects favoring theanine plus caffeine on attention-switching accuracy and digit-vigilance accuracy in the second hour, with confidence intervals that were often wide. 17
Our honest read: L-theanine is a generally well-tolerated ingredient with a real but modest research base for attention-switching accuracy and alertness when taken with caffeine. It is in Free Will at a dose that matches the research, and it adds no sugar and no stimulant.
Safety, and who should skip it
Tolerability. L-theanine has a long history of consumption in tea, and one manufacturer's L-theanine has a 2007 GRAS notice on file with FDA at up to 250 mg per serving in specified foods. 16 Reviews describe it as generally well tolerated while noting that long-term human safety data remain limited. 2 In the 250 mg trial, theanine alone produced higher "headache" ratings than placebo and slightly worse performance on a mental-arithmetic task; those effects were not seen when it was combined with caffeine. 8
Two servings a day. The label's maximum of two servings gives 500 mg of theanine and 300 mg of caffeine. The trials above used single doses of 50–250 mg or 200 mg daily; none tested 500 mg a day in this kind of formula. That is one reason to treat two servings as a ceiling, not a routine.
Blood pressure. In a 48-person study, 200 mg theanine blunted the rise in blood pressure caused by 250 mg caffeine, an experimental finding, not a treatment effect. If you take blood-pressure medication, mention Free Will to your prescriber. 13
Pregnancy, nursing, under 18. Free Will is not recommended for children, people under 18, or anyone pregnant or nursing; that is on the label. The evidence does not establish that this formula is suitable for those groups.
Caffeine. Each serving also carries 150 mg of caffeine; two servings are 300 mg before counting coffee, tea or anything else. See the caffeine page for the full picture.
What the research shows
Grades are our editorial read of the human evidence for each specific outcome, not a regulatory rating. MODERATE = several randomized trials pointing the same way, or a meta-analysis. LIMITED = one or two small trials, or mixed results. NOT SHOWN = tested and not found. MECHANISM = a measured biological effect, not a felt benefit. Study details are quoted from published abstracts unless a reference says "full text read".
L-theanine with caffeine
| Outcome | Evidence | Grade |
|---|---|---|
| Alertness and attention-switching accuracy in the first two hours | 2014 review of 11 acute studies of tea constituents: moderate effect sizes for theanine plus caffeine on Bond-Lader alertness and attention-switching accuracy; smaller effects on some other attention measures. 12 2025 meta-analysis of trials in healthy participants: small-to-moderate effects favoring the combination on attention-switching accuracy and digit-vigilance accuracy in the second hour; other outcomes had wide confidence intervals. 17 Lower-dose trials (97–100 mg theanine with 40–50 mg caffeine; 27 and 44 participants) found faster and more accurate task switching and, in one, higher alertness and lower tiredness, 9,10 while another found no change in alertness or intersensory attention, and visual search, choice reaction time and mental rotation did not change. 10,11 | MODERATE |
| Reaction time, working-memory speed, sentence verification at 250/150 mg | One 24-person crossover trial at Free Will's doses: versus placebo, the combination improved simple reaction time, numeric working-memory reaction time, sentence-verification accuracy and rapid visual information processing, and lowered "tired" and "headache" ratings. 8 | LIMITED |
| Brain activity during attention tasks | EEG: the 100/50 mg combination increased hit rate and target discrimination and lowered background alpha power during a visual attention task; theanine alone had no behavioral effect at that dose. 7 | MECHANISM |
| Reducing caffeine "jitters" | In 48 adults, 200 mg theanine did not significantly change the jitteriness, alertness or mood produced by 250 mg caffeine. It did blunt caffeine's blood-pressure rise. 13 | NOT SHOWN |
L-theanine on its own
| Outcome | Evidence | Grade |
|---|---|---|
| Stress response in the lab | 12 people doing mental arithmetic under time pressure: theanine lowered heart-rate and salivary immunoglobulin-A responses to the stressor versus placebo, a pattern the authors attributed to less sympathetic activation. 5 A 2020 systematic review of 9 trials concluded 200–400 mg/day "may assist in the reduction of stress and anxiety in people exposed to stressful conditions", and called for larger, longer studies. 15 | LIMITED |
| Four weeks of daily use | 30 healthy adults, 200 mg/day for 4 weeks, crossover: stress-symptom and sleep-quality questionnaire scores improved from baseline during the theanine period; against placebo, the differences reached significance for some sleep sub-scores and, in the lower-scoring half of participants, for verbal fluency. 14 | LIMITED |
| Alpha brain-wave activity ("relaxed alertness") | 50 mg increased alpha-band EEG activity over 45–105 minutes versus placebo in resting adults, replicated in a second group. 6 | MECHANISM |
| Cognitive performance alone | Mixed and task-specific. The 2025 meta-analysis found a small-to-moderate improvement in choice reaction time in the first hour versus placebo. 17 At 250 mg alone, theanine slightly reduced mental-arithmetic accuracy and raised headache ratings; 8 at 100 mg alone it had no effect on an attention task; 7 200 mg slowed reaction time on a visual-probe task in the 48-person study. 13 | MIXED |
Not covered here on purpose: studies in people with diagnosed psychiatric or medical conditions. Free Will is a dietary supplement, not a treatment, and those studies don't tell you what a scoop will do for a healthy adult.
How it works
Theanine is chemically close to glutamate, the brain's main excitatory neurotransmitter, and has been shown to block the binding of glutamic acid to glutamate receptors. That is the proposed explanation for its calming effect in lab studies; the exact mechanism in people is not settled. 5 After you swallow it, it is absorbed quickly and broken down mainly into ethylamine and glutamic acid; researchers note that any of the three could be responsible for its effects. 4
One of the more consistently measured effects is on brain rhythms. Doses as low as 50 mg increased alpha-band activity on EEG, a pattern associated with relaxed wakefulness rather than drowsiness. 6 When theanine was combined with caffeine during a demanding attention task, background alpha power dropped while accuracy rose, which the authors read as attention being deployed more efficiently. 7 How those EEG changes translate into what you feel is still an open question.
History
Tea has been drunk for thousands of years, but the amino acid behind much of its savory taste was only identified in 1949. Chemist Yajiro Sakato, working on the components of Japanese green tea, described a new amide he named theanine and identified as L-glutamic acid γ-ethylamide; his 1950 report measured about 1% theanine in shaded gyokuro tea. 1 Shading the plant before harvest is still how growers raise theanine content today, which is why gyokuro and ceremonial-grade matcha are often the richest sources. 3
FDA responded to a GRAS notice for purified L-theanine in 2007. 16 The research pairing it with caffeine, the reason it is in Free Will, is almost entirely from 2007 onward. 7,8,9,10,11,13
Questions people ask
Does L-theanine cancel out caffeine?
Not in the way people hope. In the one study that tested it directly, 200 mg theanine did not reduce the jitteriness or alter the alertness that 250 mg caffeine produced; it did blunt caffeine's rise in blood pressure. 13 What the combination studies show is a different profile from placebo: better attention-switching accuracy and alertness in the first couple of hours. 12,17
How much L-theanine is in a cup of tea?
Far less than a scoop of Free Will. A cup of black tea contains roughly 20 mg; 6 the 1950 report measured about 1% theanine in gyokuro leaf, and shaded teas are often the richest. 1,3 You would need many cups to reach 250 mg.
Why 250 mg of L-theanine and 150 mg of caffeine?
Because those are the doses tested together in the Haskell 2008 crossover trial. 8 Lower-dose studies used roughly 100 mg theanine with 50 mg caffeine and found similar attention-switching effects. 9,10 No study has established an optimal ratio.
Can you take L-theanine every day?
A four-week crossover trial gave 30 healthy adults 200 mg per day; 14 a 2007 GRAS notice covers one manufacturer's L-theanine at up to 250 mg per serving. 16 Free Will's label caps you at two servings a day, which is 500 mg of theanine and 300 mg of caffeine, a level no trial has tested in this kind of formula.
Is the L-theanine in Free Will from tea?
Purified L-theanine is sold both as a tea extract and as a synthesized ingredient; they are the same molecule. We will publish the source and manufacturer of ours once the first production batch and its documentation are in hand. We won't claim a source we can't yet show.
Does L-theanine help you sleep?
Free Will is not a sleep product; it contains 150 mg of caffeine. In a four-week trial of theanine alone, some sleep-quality sub-scores improved versus placebo in 30 healthy adults, a preliminary finding. 14 Take Free Will earlier in the day and keep caffeine away from bedtime.
References
Every source below was opened before it was linked. Study sizes, doses and results are quoted from the published abstracts, except where a reference says "full text read".
- Sakato Y. Research on tea components (report 3): a new amide, "theanine". Nippon Nōgeikagaku Kaishi 1950;23(6):262–267. doi:10.1271/nogeikagaku1924.23.262 (Japanese; abstract read).
- Türközü D, Şanlier N. L-theanine, unique amino acid of tea, and its metabolism, health effects, and safety. Crit Rev Food Sci Nutr 2017. PubMed 26192072.
- Unno K, et al. Compositional diversity and functional interactions of bioactive compounds in Japanese green tea: focus on matcha, sencha, and gyokuro. Molecules 2026. PubMed 42654013.
- Scheid L, et al. Kinetics of L-theanine uptake and metabolism in healthy participants are comparable after ingestion of L-theanine via capsules and green tea. J Nutr 2012. PubMed 23096008.
- Kimura K, et al. L-Theanine reduces psychological and physiological stress responses. Biol Psychol 2007. PubMed 16930802.
- 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. PubMed 18296328.
- Kelly SP, et al. L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance. J Nutr 2008. PubMed 18641209.
- Haskell CF, Kennedy DO, Milne AL, Wesnes KA, Scholey AB. The effects of L-theanine, caffeine and their combination on cognition and mood. Biol Psychol 2008;77(2):113–122. PubMed 18006208. Full text read.
- Owen GN, et al. The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutr Neurosci 2008. PubMed 18681988.
- Giesbrecht T, et al. The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness. Nutr Neurosci 2010. PubMed 21040626.
- Einöther SJ, et al. L-theanine and caffeine improve task switching but not intersensory attention or subjective alertness. Appetite 2010. PubMed 20079786.
- Camfield DA, et al. Acute effects of tea constituents L-theanine, caffeine, and epigallocatechin gallate on cognitive function and mood: a systematic review and meta-analysis. Nutr Rev 2014. PubMed 24946991.
- Rogers PJ, et al. Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together. Psychopharmacology 2008. PubMed 17891480.
- Hidese S, et al. Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomized controlled trial. Nutrients 2019. PubMed 31623400.
- Williams JL, et al. The effects of green tea amino acid L-theanine consumption on the ability to manage stress and anxiety levels: a systematic review. Plant Foods Hum Nutr 2020. PubMed 31758301.
- U.S. FDA. GRAS Notice No. 209, L-theanine, submitted by Taiyo International; FDA response "no questions", February 5, 2007; notified use up to 250 mg per serving in specified food categories. FDA GRAS inventory.
- Effects of tea (Camellia sinensis) or its bioactive compounds L-theanine or L-theanine plus caffeine on cognition, sleep, and mood in healthy participants: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev 2025. PubMed 40314930.
- Unno K, et al. Effects of Japanese green tea on the brain (in Japanese). Brain Nerve 2026. PubMed 41974439.
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