A summary of published research, not medical advice. See the Disclaimer.
Studies: every indexed trial and review on this topic, scored
Caffeine and L-theanine is a pairing of the stimulant in coffee and tea with an amino acid found mainly in tea leaves. Trials gave the two together as a single dose.
Best-supported effect: Alertness and accuracy when switching between tasks, within about two hours of one dose, in healthy adults. Probably helps (moderate evidence).
Most-cited weak claim: L-theanine takes the edge off caffeine's jitteriness. Unknown (very weak evidence).
How we grade evidence · Last reviewed: October 2026
What you need to know
In healthy adults, one dose of caffeine with L-theanine improved alertness and accuracy when switching between tasks for about two hours, with moderate evidence. Caffeine alone improves alertness and reaction time a little, and improves exercise performance, both with strong evidence. Whether L-theanine takes the edge off caffeine's jitteriness is unknown: no trial measured it directly, and no combination trial followed people past a single test day. Caffeine raised blood pressure (moderate evidence), and in blinded challenge studies about half of people with panic disorder had a panic attack after 400 to 750 mg. A caffeine safety review named women of reproductive age and children as at-risk groups. Trials of the combination used 40 to 150 mg caffeine with 97 to 250 mg L-theanine as a single dose.
| Effect | Verdict | What was measured | Studied in |
|---|---|---|---|
| Alertness and reaction time from caffeine alone | Helps a little (strong evidence) Reviews and a pooled analysis in sport show consistent results in healthy adults |
Measured | Healthy adults, including people in sport and under restricted sleep |
| Exercise performance from caffeine alone | Helps (strong evidence) Expert society's review found benefits across many types of exercise |
Measured | Trained and untrained individuals |
Caffeine alone at low to moderate doses improved alertness, vigilance, attention and reaction time, with less consistent effects on memory and decision making. Repeated doses helped maintain performance during restricted sleep [5]. A 2021 meta-analysis of caffeine in sport found improvements in attention, accuracy and speed [6]. Helps a little (strong evidence): consistent results in the populations named.
A position stand found small to moderate benefits of caffeine across many types of exercise. Benefits were most consistent in aerobic endurance and differed between individuals [7]. Helps (strong evidence).
Shown in humans
| Side effect | Verdict | How often vs placebo | Source | Studied in |
|---|---|---|---|---|
| General toxicity, heart, bone or behaviour effects from caffeine up to 400 mg/day in healthy adults | May not increase it (weak evidence) One safety review in healthy adults, no placebo rates reported |
Not reported | [10] | Healthy adults |
| More side effects at very high caffeine doses (such as 9 mg/kg) | May happen (weak evidence) One expert society statement reports this, with no placebo rates |
Not reported | [7] | Trained and untrained individuals |
| Rise in blood pressure after caffeine | Probably happens (moderate evidence) Review of pooled analyses, no frequency against placebo given |
Not reported | [16] | |
| Panic attacks after 400–750 mg caffeine in people with panic disorder | Probably happens (moderate evidence) Blinded challenge studies, 51% vs 0% across 237 patients in 9 studies |
51% vs 0% (237 patients); 1.7% of healthy controls after caffeine | [15] | People with panic disorder |
| Shorter, more disturbed sleep after 400 mg caffeine up to 6 hours before bed | May happen (weak evidence) One placebo-controlled sleep trial, no rates given |
Not reported | [14] | |
| Pregnancy loss linked to caffeine intake | Unknown (very weak evidence) Link comes only from pooled studies that followed people over time, not trials |
Not reported | [16] |
The caffeine review suggested a lower limit of 300 mg/day for women of reproductive age [10]. The pregnancy loss link comes from meta-analyses of observational studies [16].
This lists what studies and labels report. A pharmacist or prescriber can say whether it applies to a specific person.
| Combined with | What happened | Verdict | Source | Studied in |
|---|---|---|---|---|
| Fluvoxamine | Caffeine clearance fell from 105 to 9.1 mL/min and half-life rose from 4.9 to 56 hours in 7 healthy adults. Alertness and psychomotor effects of a single 250 mg caffeine dose did not change. | May happen (weak evidence) One trial in 7 healthy adults, caffeine cleared more slowly |
[18] | 7 healthy adults |
| Ciprofloxacin | Caffeine half-life rose from 5.2 to 8.2 hours and exposure by about 60% in 10 healthy men | May happen (weak evidence) One trial in 10 healthy men, caffeine took longer to fall by half |
[19] | 10 healthy men |
| Oestrogen-containing oral contraceptives | Caffeine half-life was 7.9 hours in long-term users vs 5.4 hours in matched non-users, with lower clearance | May happen (weak evidence) One comparison study of long-term users and matched non-users |
[20] |
| Claim | Verdict | What was measured | Studied in |
|---|---|---|---|
| L-theanine alone lowers stress and anxiety | May help a little (weak evidence) Review of nine trials with random assignment, mainly in people under stress |
Self-reported | People exposed to stressful conditions |
A 2020 systematic review of nine randomised trials reported that 200–400 mg/day of L-theanine alone may reduce stress and anxiety, mainly in people exposed to stressful conditions [8]. May help a little (weak evidence): the authors called for longer and larger trials.
Shown in humans
Shown only in animals or cells
| Side effect | Verdict | How often vs placebo | Source | Studied in |
|---|---|---|---|---|
| Headache from L-theanine alone (the combination lowered headache ratings) | May happen (weak evidence) One trial where people tried each option, no one knew which, combination lowered ratings instead |
Not reported | [4] | |
| Harm from high-dose L-theanine | Unknown (very weak evidence) Rat toxicity data only, no human studies |
Not studied in humans; no adverse effects in rats at up to 4,000 mg/kg/day for 13 weeks | [17] | Rats |
The headache row comes from a crossover trial that tested L-theanine alone, caffeine alone and the combination [4].
No interaction studies of L-theanine with drugs or supplements were found.
| Effect | Verdict | What was measured | Studied in |
|---|---|---|---|
| Alertness and task-switching accuracy after one dose of the combination | Probably helps (moderate evidence) Pooled analysis of 11 small single-dose trials, results varied by task |
Measured | Healthy adults, single doses |
A 2014 meta-analysis of 11 placebo-controlled single-dose trials found moderate effects of the combination on self-rated alertness and on accuracy in tasks that require switching attention, with smaller effects on some other attention measures, within two hours of the dose [1]. Placebo-controlled trials in 27 and 44 adults found better task-switching accuracy and alertness at 40–50 mg caffeine plus about 100 mg L-theanine [2][3]. A crossover trial at higher doses found higher alertness ratings and faster reaction times [4]. Results on other tasks were less consistent [1][3]. Probably helps (moderate evidence): downgraded because samples were small, every trial tested a single dose, and results varied by task. Researchers from Unilever's research division ran two of the trials [2][3].
| Claim | Verdict | What was measured | Studied in |
|---|---|---|---|
| L-theanine reduces caffeine's jitteriness or anxiety | Unknown (very weak evidence) No trial measured this directly, mood ratings were not a main outcome |
Self-reported | Healthy adults, mood ratings as secondary outcomes |
| Attention in children with ADHD | Unknown (very weak evidence) One trial in five boys who tried each option, not repeated |
Measured | Five boys with ADHD |
| Lasting cognitive improvement from daily use | Unknown (not tested) No combination trial followed people past a single test day |
Measured | No trials |
The jitteriness claim rests on L-theanine's reported calming effects and on how the pairing is marketed. The combination trials recorded self-rated mood, but none was designed to test this claim. Two trials included a caffeine-only group, and what L-theanine added beyond caffeine varied by task [2][4]. In the meta-analysis, effects tracked caffeine dose more closely than L-theanine dose [1]. Unknown (very weak evidence): no trial measured it directly.
The ADHD claim rests on one proof-of-concept crossover trial in five boys. After the combination, overall cognition scores and Go/NoGo performance improved, inhibitory control showed a non-significant trend toward improvement, and activity fell in a brain network linked to mind wandering [9]. It has not been replicated. Unknown (very weak evidence).
No combination trial followed people past a single test day, so daily use over weeks or months is untested.
Theory only
The combination trials used single doses and were not designed to detect harms. They did not test interactions with medications.
This lists what studies and labels report. A pharmacist or prescriber can say whether it applies to a specific person.
These are the amounts used in the studies. They are not recommendations.
Trials of the combination in adults used 40–150 mg caffeine with 97–250 mg L-theanine as a single dose. Testing ran from about 20 minutes to 2 hours after the dose [1][2][3][4]. Trials of L-theanine alone for stress used 200–400 mg/day [8]. Exercise trials of caffeine alone used 3–6 mg/kg, most often about 60 minutes before exercise [7].