What Is L-Theanine and What Does It Do? The Trials and the UK Rules

L-theanine is an amino acid found almost only in tea leaves: a 200 mL cup of black or green tea carries about 12 mg of it, while the supplement trials used 200 mg a day.¹ At that dose, 30 healthy adults over four weeks had lower trait-anxiety and sleep-quality scores than on placebo; across 18 trials of 897 people, self-rated sleep quality improved by a small to moderate amount (standardised mean difference 0.43); and a nine-trial review found 200 to 400 mg a day may reduce stress and anxiety under stressful conditions.^{2, 3, 4} With caffeine, 100 mg of theanine improved attention-switching speed and accuracy in 27 volunteers, and 97 mg improved task-switching accuracy in 44 young adults.^{6, 7} European food-safety assessors found its proposed claims unproven in 2011, so no health claim may be made for it in Great Britain.^{8, 9} It sits beside the Wellgard ashwagandha guide.

What is L-theanine, and which foods contain it?

L-theanine (gamma-glutamylethylamide) is a non-protein amino acid made by the tea plant, Camellia sinensis, where it gives the brew its savoury, brothy taste. It is not in coffee, cocoa or herbal infusions. A 2025 analysis brewed six tea types and found black and green tea highest, at about 12.3 and 12.5 mg per 200 mL cup after a standard steep; white, oolong and fermented teas carried less.¹ So "which foods are rich in L-theanine" has a one-word answer, tea, and the 200 mg trial dose is roughly 16 cups.

What does L-theanine do in the body?

It crosses into the brain and blocks glutamate, the main excitatory messenger, at some of its receptors, and the effect shows in brain electrical activity: healthy adults given a theanine drink had more resting alpha-wave activity two hours later, though only those higher in trait anxiety.¹⁰ In 12 students doing mental arithmetic, theanine blunted the rise in heart rate and salivary immunoglobulin A, with heart-rate variability pointing to a quieter sympathetic nervous system.¹¹ In 14 adults it held down the blood-pressure rise a mental task produced in those who reacted most, and lowered tension-anxiety scores.¹² None of these three studies, of 34, 12 and 14 people, measured sedation or drowsiness; the changes were in alpha activity, heart rate and blood pressure during a task.^{10, 11, 12}

Does L-theanine help with stress and anxiety?

The best single trial is Japanese: 30 adults, average age 48, took 200 mg of L-theanine or placebo for four weeks each in a crossover design, and depression, trait-anxiety and Pittsburgh Sleep Quality Index scores fell on theanine (p = 0.019, 0.006 and 0.013), with verbal fluency also better; the tablets came from a theanine manufacturer, which the paper declares.² A 2020 systematic review of nine trials of pure theanine found 200 to 400 mg a day may reduce stress and anxiety in people exposed to stressful conditions, and asked for longer, larger trials.⁴ In 34 adults given a theanine drink before a multitasking stressor, subjective stress was lower at one hour and the salivary cortisol response lower at three.¹⁰ It has not been compared with any prescribed care: panic attacks or a suspected anxiety disorder start with a GP and NHS talking therapies.

Does L-theanine help you sleep?

The 2025 meta-analysis in Sleep Medicine Reviews pooled 18 randomised trials of 897 people: self-rated time to fall asleep improved (standardised mean difference 0.15, small), as did daytime dysfunction (0.33) and overall sleep quality (0.43), the authors noting how few trials used theanine alone and that dose and duration are undefined.³ In the four-week trial the sleep-latency, sleep-disturbance and sleep-medication subscales of the PSQI beat placebo.² No laboratory sleep recording exists for theanine alone, so these are questionnaire effects, modest and consistent. Magnesium's three trials in 151 older adults found about 17 minutes faster sleep onset at low to very low quality of evidence (see does magnesium help you sleep).¹³

What does L-theanine do with caffeine?

In a UK crossover study of 27 volunteers, 50 mg of caffeine with 100 mg of theanine improved speed and accuracy on an attention-switching task at 60 minutes and reduced distraction in a memory task, where caffeine alone improved only alertness.⁶ In 44 young adults, 97 mg of theanine with 40 mg of caffeine improved task-switching accuracy and alertness and reduced tiredness.⁷ At 250 mg with 150 mg of caffeine, reaction times sped up and alertness rose, while theanine alone at 250 mg increased headache ratings and worsened a serial-subtraction task: more is not better.⁵ A cup of tea already contains the pairing, at a tenth of the trial doses.

Why is there no L-theanine health claim in the UK?

In June 2011 the European Food Safety Authority's nutrition panel assessed the health claims submitted for L-theanine from tea, covering cognitive function, psychological stress, sleep and menstrual comfort, and concluded that a cause-and-effect relationship had not been established for any of them.⁸ Great Britain carried that register over in 2020 and keeps it on gov.uk; theanine has no authorised entry.⁹ So a UK label may not say theanine calms, relaxes, helps sleep or sharpens focus; it may state the amount. That is a verdict on the 2011 dossiers, not a finding that the ingredient is inert; most of the trials above came later. The four-week trial used 200 mg a day.² Sleep powders that pair theanine with magnesium carry their authorised wording on the magnesium: it contributes to normal psychological function, normal functioning of the nervous system and the reduction of tiredness and fatigue.

What dose did the trials use, and is L-theanine safe?

200 mg a day is the dose with the most evidence, 200 to 400 mg the range the reviews cover, and 100 mg with caffeine the attention dose.^{4, 6} Safety data are reassuring within their limits: trials of a few weeks reported no serious adverse events, the commonest complaint was headache at 250 mg alone, and a 13-week feeding study in rats found no adverse effect at the highest dose tested, 4,000 mg per kilogram a day.^{5, 14} Nobody has run a long human trial. What not to mix, with no interaction study to go on: sedatives, sleeping tablets and antidepressants, where the label caution is to tell the pharmacist or prescriber; blood-pressure medicines, because theanine blunted a stress-related pressure rise in one small study and effects could add; and large doses of stimulants.¹² Pregnancy and breastfeeding have no supplement data, so labels say ask a GP; it is not for children. If it makes you drowsy, do not drive until you know how it affects you.

Is ashwagandha better than L-theanine, or magnesium?

They were tested for different things in different people, so the comparison has no trial behind it. Ashwagandha: 600 mg a day of root extract for 60 days lowered perceived stress and serum cortisol (about 28% against 8% on placebo) in 64 stressed adults, with pooled analyses rated low certainty and two recent well-run trials null; it also raised thyroxine in pooled data and carries a liver caution, set out in the guide.¹⁵ L-theanine: 200 mg for four weeks in 30 healthy adults, small effects on trait anxiety and sleep quality, no organ cautions.² Magnesium: about 17 minutes faster sleep onset in 151 older adults, low to very low quality evidence.¹³ Taking them together has not been trialled; anyone on a prescribed medicine for anxiety or sleep tells the pharmacist first.

Frequently asked questions

Does L-theanine raise cortisol levels?

No trial has found a rise. The one study that measured salivary cortisol found the response to a stressor lower three hours after a theanine drink.¹⁰

Is there a downside to L-theanine?

Headache at 250 mg taken alone in one trial, drowsiness in some people's reports, and no UK claim on the label.⁵ Serious harm has not been reported in trials.

How long does L-theanine take to work?

Heart-rate, blood-pressure and attention effects appeared within one to two hours of a dose; the anxiety and sleep-quality changes were measured after four weeks of daily use.^{2, 6, 11}

Can you take L-theanine every day?

The four-week trial gave it daily without reported harm; beyond a few weeks there is no human data, a limit of the evidence rather than a warning.^{2, 3}

Sources

1. Ayakdaş G, Ağagündüz D. Determination of L-theanine and caffeine contents in tea infusions with different fermentation degrees and brewing conditions using the chromatographic method. Foods, 2025. PubMed 40647065.
2. 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.
3. Bulman A, et al. The effects of L-theanine consumption on sleep outcomes: a systematic review and meta-analysis. Sleep Medicine Reviews, 2025. PubMed 40056718.
4. 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 for Human Nutrition, 2020. PubMed 31758301.
5. Haskell CF, et al. The effects of L-theanine, caffeine and their combination on cognition and mood. Biological Psychology, 2008. PubMed 18006208.
6. Owen GN, et al. The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutritional Neuroscience, 2008. PubMed 18681988.
7. Giesbrecht T, et al. The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness. Nutritional Neuroscience, 2010. PubMed 21040626.
8. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on the substantiation of health claims related to L-theanine from Camellia sinensis (L.) Kuntze (tea). EFSA Journal, 2011;9(6):2238. doi.org/10.2903/j.efsa.2011.2238.
9. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. gov.uk.
10. White DJ, et al. Anti-stress, behavioural and magnetoencephalography effects of an L-theanine-based nutrient drink: a randomised, double-blind, placebo-controlled, crossover trial. Nutrients, 2016. PubMed 26797633.
11. Kimura K, et al. L-theanine reduces psychological and physiological stress responses. Biological Psychology, 2007. PubMed 16930802.
12. Yoto A, et al. Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses. Journal of Physiological Anthropology, 2012. PubMed 23107346.
13. Mah J, Pitre T. Oral magnesium supplementation for insomnia in older adults: a systematic review and meta-analysis. BMC Complementary Medicine and Therapies, 2021. PubMed 33865376.
14. Borzelleca JF, et al. A 13-week dietary toxicity and toxicokinetic study with L-theanine in rats. Food and Chemical Toxicology, 2006. PubMed 16759779.
15. Chandrasekhar K, et al. A prospective, randomized double-blind, placebo-controlled study of safety and efficacy of a high-concentration full-spectrum extract of ashwagandha root in reducing stress and anxiety in adults. Indian Journal of Psychological Medicine, 2012. PubMed 23439798.

General information, not medical advice: on a sedative, sleeping tablet, antidepressant or blood-pressure medicine, ask a pharmacist before adding L-theanine, and never change a prescribed medicine for a supplement.