Evidence explainerL-theanineCaffeine (capsules, gum and anhydrous powder)
L-Theanine and Caffeine: What the "Calm Focus" Stack Actually Did in Trials
The pairing comes from tea and is now a supplement staple. Trials show small gains in attention and alertness over a couple of hours, and theanine alone has its own modest attention signal.
Key takeaways
- In pooled trials, theanine plus caffeine gave small to moderate gains in attention and mood in the first 2 hours versus placebo [1].
- In one trial, 97 mg of L-theanine with 40 mg of caffeine improved task-switching accuracy and alertness, but not choice reaction time or visual search [2].
- On its own, a single 200 mg dose of L-theanine improved choice reaction time across 31 trials [3].
- Combination trials cannot separate theanine's effect from caffeine's [2].
- Combination products carry caffeine's side effects; FDA says 400 mg/day of caffeine is not usually dangerous for healthy adults [4].
Safety at a glance
From our L-theanine monograph. Published interactions and cautions only, each with its source.
Interactions
Who should be careful
- Limited safety data. No serious adverse events were reported across 31 randomized trials, but most were short. Data on pregnancy, breastfeeding and long-term use were not reported in our sources.[1]
- Combination products. Many products pair theanine with caffeine; check the caffeine content.[2]
- Before you start. Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
The evidence, graded
Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.
- Grade DImprovedAttention and alertness with caffeineThe combination improved accuracy during task switching and self-reported alertness, and reduced tiredness. Visual search, choice reaction time and mental rotation did not change.
- Grade BImprovedAttention (choice reaction time) and acute stressA single 200 mg dose improved choice reaction time (SMD 0.51). Acute stress fell modestly (SMD 0.31), mostly driven by studies at high risk of bias; fatigue did not change, and anxiety effects were inconsistent. No serious adverse events were reported.
Why the two are paired
Both occur naturally in tea [1]. Supplement makers combine them on the idea that theanine smooths caffeine's edge while caffeine adds alertness. A 2025 systematic review of tea, theanine and theanine-plus-caffeine trials in healthy people is the broadest look at that idea [1].
What the combination trials found
The 2025 review found theanine with caffeine produced small to moderate improvements in attention and mood measures in the first 2 hours compared with placebo [1]. The authors called for more research using tea-equivalent doses [1].
One 2010 trial in 44 young adults gave 97 mg of L-theanine with 40 mg of caffeine and tested them at 20 and 70 minutes [2]. Task-switching accuracy and self-rated alertness improved and tiredness fell, while visual search, choice reaction time and mental rotation did not change [2]. Because it tested only the combination, it cannot show what theanine adds on top of caffeine [2].
Theanine on its own
A 2026 meta-analysis of 31 placebo-controlled trials (1,168 participants) found a single 200 mg dose taken 30 to 60 minutes before testing improved choice reaction time, a measure of attention (SMD 0.51) [3]. No serious adverse events were reported [3].
For stress, anxiety and sleep, the theanine evidence is weaker; see L-theanine for anxiety and sleep.
Do not forget the caffeine
A theanine-caffeine capsule is still a caffeine product. Heavy use of 500 mg/day or more can disturb sleep and cause irritability and anxiety, and FDA says 400 mg/day does not usually have dangerous effects in healthy adults [4]. Add up all sources; see how much caffeine is too much.
Quick answers
What ratio of L-theanine to caffeine was studied?
One trial used 97 mg of L-theanine with 40 mg of caffeine [2]. Products vary, and our sources do not establish a best ratio [1].
Does L-theanine cancel caffeine jitters?
The trials measured attention, mood and alertness, not jitters directly, so our sources cannot confirm that claim [1][2].
Is green tea the same as the supplement stack?
Tea contains both, plus polyphenols, but few trials used tea-equivalent doses [1].
Keep reading
Full monographs
Related articles
- L-Theanine for Anxiety and Sleep: The Evidence Is Thinner Than the Marketing
- Caffeine Before a Workout: 3 mg/kg, 15 to 60 Minutes Out, and Why It Helps Endurance Most
- How Much Caffeine Is Too Much? 400 mg a Day, 100 mg for Teens, and Why Pure Powder Is Dangerous
- Is the Taurine in Energy Drinks Bad for You? The Safety Data Points at the Caffeine
Browse the full Supplement & Nutrient Encyclopedia or all supplement articles.
References
- [1]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.
- [2]The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness. Nutr Neurosci, 2010.
- [3]Cognitive and affective effects of L-Theanine: a systematic review and meta-analysis of 31 randomized trials. Mol Psychiatry, 2026.
- [4]Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2024.