L-theanine: what it does, and how it compares with ashwagandha
What L-theanine is, what the research shows for focus, stress and sleep, how much to take, and how it compares with ashwagandha.
- Written by
- Amy D
- Reviewed by
- Nooroots team
- Last updated
- Reading time
- 7 min read
Overview
L-theanine is an amino acid found in tea. Its best-supported effect is short-term: in trials, a single 200 mg dose sharpened attention within an hour, and it pairs well with caffeine. Evidence for stress is mixed and for sleep is early. Trials use 200 to 400 mg a day, and no serious side effects have been reported.
Key points
- What it is: an amino acid in tea; a cup of black tea has about 24 mg.
- Focus: one 200 mg dose sped up reaction times in trials, within an hour.
- Stress and sleep: results are mixed for stress and early for sleep.
- Dose: trials use 200 mg at a time, or 200 to 400 mg a day.
- Vs ashwagandha: L-theanine is tested within an hour; ashwagandha over 8 weeks or more.
What is L-theanine?
L-theanine is an amino acid found naturally in tea leaves, and you can also take it as a supplement [6]. It's one of the best-known natural nootropics, often paired with caffeine.
A cup of tea gives you much less than a supplement. In a UK test of 27 supermarket teas, a cup of black tea held about 24 mg of L-theanine and green tea about 8 mg [6]. So one 200 mg capsule is roughly what's in 8 cups of black tea.
Brewing for longer released more L-theanine, and a splash of milk or sugar made no real difference [6].
What are the benefits of L-theanine?
Its best-supported effect is short-term: sharper attention for an hour or two after a dose. Across 31 trials with 1,168 people, a single 200 mg dose taken 30 to 60 minutes before testing gave faster reaction times [1].
Results for stress and sleep are less clear, as the table shows.
| Use | What trials found | Evidence |
|---|---|---|
| Attention, one dose | Faster reaction times 30 to 60 minutes after 200 mg, across 31 trials [1] | Moderate |
| Stress | A small effect after one dose, mostly from weaker studies [1]; a review of 9 trials says 200 to 400 mg a day "may assist" [4] | Mixed |
| Sleep | Small gains in self-rated sleep across 19 studies, few of them using L-theanine alone [2] | Early |
Does L-theanine work with caffeine?
It seems to pair well with it. A 2025 review of trials in healthy people found that L-theanine with caffeine improved accuracy when switching between tasks, in the second hour after a dose Moderate [3].
Tea naturally contains both, so every cup gives you a little of each. For more on pairing ingredients, see our guide to nootropic stacks.
Does L-theanine help you sleep?
It may help a little. A 2025 review of 19 studies with 897 people found small improvements in self-rated sleep quality, time to fall asleep and how people felt the next day [2].
In 30 healthy adults with an average age of 48, 200 mg at bedtime for 4 weeks improved some sleep scores compared with a placebo Early [5]. For other options, see magnesium for sleep and our guide to melatonin in the UK.
How much L-theanine per day?
Trials usually give 200 mg as a single dose, or 200 to 400 mg a day for a few weeks [1][4][5]. That's far more than you'd get from tea.
If you're trying it for focus, the trial dose is 200 mg. For sleep, trials have used 200 mg at bedtime [5].
How long does L-theanine take to work?
About 30 to 60 minutes for focus, which is when trials test attention after a single dose [1]. Sleep trials have run for about 4 weeks [5].
Its effect on attention seems to be short-lived. In 50 adults aged 50 to 69, one 100 mg dose sped up an attention task, but after 12 weeks of daily use there was no effect on attention Early [10].
L-theanine side effects
L-theanine has a good safety record in trials: none of 31 trials reported a serious side effect [1]. Most lasted from a single dose to a few weeks, so less is known about taking it for months.
It hasn't been studied in pregnancy or alongside regular medicines, so ask your GP or pharmacist first if either applies to you.
L-theanine vs ashwagandha
They work on different timescales. L-theanine is tested an hour after a single dose, while ashwagandha is usually taken every day for 8 weeks or more [1][9]. No trial has compared them head to head.
| L-theanine | Ashwagandha | |
|---|---|---|
| What it is | An amino acid from tea leaves [6] | An extract of the root of Withania somnifera, a plant used in Ayurveda |
| How fast it acts | Tested 30 to 60 minutes after one dose [1] | Over weeks; sleep effects were larger after 8 weeks or more [9] |
| Typical trial dose | 200 mg as one dose, or 200 to 400 mg a day [1][4] | 250 to 600 mg of root extract a day [8][9] |
| Best-studied for | Attention after a single dose [1] | Cortisol, the main stress hormone, and sleep [8][9] |
For ashwagandha, pooled trials found lower cortisol Moderate [8] and better sleep scores Moderate [9]. How stressed people said they felt didn't clearly change Mixed [8]. Our ashwagandha guide has the detail.
The nearest thing to a head-to-head test was a 2-week trial of two blends. A morning blend with ashwagandha and rhodiola lowered weekly stress slightly against a placebo, while an evening blend with 200 mg of L-theanine didn't Early [7].
Where Nooroots fits
Nooroots Ashwagandha + Lion's Mane doesn't contain L-theanine. It gives you 120 mg of KSM-66 ashwagandha and 500 mg of lion's mane extract in two caffeine-free capsules a day.
It's made for calm focus and calm resilience on demanding days, as part of a morning routine. KSM-66 is a root-only extract standardised to 5% withanolides.
Frequently asked questions
When should you take L-theanine?
About 30 to 60 minutes before you need to focus, as in single-dose trials, or at bedtime if you're taking it for sleep [1][5].
What foods contain L-theanine?
Tea is the main source. Black, green and white tea all contain it, and black tea gave the most per cup in a UK test of 27 teas [6].
Is L-theanine the same as magnesium?
No. L-theanine is an amino acid from tea, while magnesium is a mineral, though some sleep blends contain both [7]. Our guide to how much magnesium per day covers magnesium.
Can you take melatonin with L-theanine?
No study has tested the two together, so how they combine isn't known. In the UK, melatonin is a prescription-only medicine; our melatonin guide explains the rules.
Can you take ashwagandha and L-theanine together?
No interaction is known, though no trial has tested the two on their own together. One 2-week trial gave both, in separate morning and evening blends [7]; see ashwagandha side effects for who shouldn't take ashwagandha.
References
Every study here is listed so you can read it yourself. This guide is for general information and doesn't replace advice from your GP or pharmacist. We review it when new research is published.
- Gerolymos C et al. (2026). Cognitive and affective effects of L-theanine: a systematic review and meta-analysis of 31 randomized trials. Molecular Psychiatry 31(11): 6592–6602. PMID 42410082
- Bulman A et al. (2025). The effects of L-theanine consumption on sleep outcomes: a systematic review and meta-analysis. Sleep Medicine Reviews 81: 102076. PMID 40056718
- Payne ER et al. (2025). 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. Nutrition Reviews 83(10): 1873–1891. PMID 40314930
- Williams JL et al. (2020). The effects of green tea amino acid L-theanine consumption on the ability to manage stress and anxiety levels: a systematic review. Plant Foods for Human Nutrition 75(1): 12–23. PMID 31758301
- Hidese S et al. (2019). Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomized controlled trial. Nutrients 11(10): 2362. PMID 31623400
- Keenan EK et al. (2011). How much theanine in a cup of tea? Effects of tea type and method of preparation. Food Chemistry 125(2): 588–594. doi:10.1016/j.foodchem.2010.08.071
- Mastrofini GF et al. (2026). Modulating stress and sleep through a multi-compound dietary supplement: a randomized, double-blind, placebo-controlled trial. Journal of the International Society of Sports Nutrition 23(1): 2740227. PMID 42829807
- Albalawi AA (2025). Dual impact of Ashwagandha: significant cortisol reduction but no effects on perceived stress. A systematic review and meta-analysis. Nutrition and Health 31(4): 1395–1408. PMID 40746175
- Cheah KL et al. (2021). Effect of Ashwagandha (Withania somnifera) extract on sleep: a systematic review and meta-analysis. PLOS ONE 16(9): e0257843. doi:10.1371/journal.pone.0257843
- Baba Y et al. (2021). Effects of L-theanine on cognitive function in middle-aged and older subjects: a randomized placebo-controlled study. Journal of Medicinal Food 24(4): 333–341. PMID 33751906
