An amino acid found in tea leaves.
Why it’s used: Gives green tea its calm, focused character; often paired with caffeine.
L-theanine is an amino acid found naturally in the leaves of the tea plant, Camellia sinensis. It is not one of the amino acids the body uses to build proteins, so there is no daily requirement for it. It is what gives green tea its calm, focused character, and it is often paired with caffeine in drinks and supplements.
Researchers have studied L-theanine in small trials looking at feelings of stress, relaxation and attention. A 2020 systematic review of nine randomised controlled trials, which used 200 to 400mg a day, reported possible effects on stress in some studies, but the trials were small and varied, and more research is needed.
In 2011 the European Food Safety Authority reviewed proposed claims linking L-theanine to cognitive function, psychological stress, normal sleep and menstrual discomfort, and concluded that a cause-and-effect relationship had not been established. That is why there are no authorised health claims for L-theanine in the UK. On labels it appears as L-theanine, sometimes alongside green tea extract or caffeine, with amounts in milligrams (mg).
3 findings behind this ingredient. Hover a card for the detail and the source.
Meta-analysis of 31 placebo-controlled randomised trials with 1,168 participants. 200 mg taken 30–60 minutes before testing improved choice reaction time (SMD 0.51); effects on acute stress were modest and no serious adverse events were reported.
Randomised, placebo-controlled, double-blind crossover trial in 30 healthy adults. Depression, anxiety and sleep quality scores fell after L-theanine, and sleep latency, sleep disturbance and use of sleep medication improved compared with placebo.
Placebo-controlled study in 44 young adults. The combination improved task-switching accuracy and self-reported alertness and reduced self-reported tiredness, with no effect on other cognitive tasks.
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L-theanine is an amino acid from tea leaves that gives green tea its calm, focused character. Researchers have studied it in small trials on stress and attention, but EFSA concluded in 2011 that the claimed effects on cognitive function, stress and sleep had not been established.
L-theanine is often paired with caffeine, and some studies have looked at the two together. If you combine them, keep track of your total caffeine from coffee, tea, energy drinks and supplements, and avoid it later in the day if caffeine affects your sleep.
Human studies have commonly used 200 to 400mg a day, and there is no official UK recommended amount. Follow the dose on the product label, and check with a pharmacist first if you take blood pressure medicines or are pregnant or breastfeeding.
Information only, not medical advice. If you take medication, are pregnant or breastfeeding, speak to your GP or pharmacist first.
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