L-theanine supplements are touted for stress relief, focus and better sleep. Although the evidence so far is preliminary, studies suggest the compound may have several brain benefits
By Isabel Baldwin
5 September 2025
Black tea and green teas including matcha contain varying levels of L-theanine
INNA FINKOVA / Alamy Stock Photo
L-theanine is an amino acid naturally found in tea leaves. First linked to tea’s calming effects in the 1940s, it is now sold as a supplement for stress relief, focus and better sleep. Research into its effects has so far been limited, but there are intriguing hints it may have several benefits for the brain, particularly in people who have anxiety.
What beneficial effects might L-theanine have?
Research into L-theanine is promising, but still in its infancy. “The research is limited, the studies are small and the results are mixed,” says Crystal Haskell-Ramsay at Northumbria University in the UK.
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However, data from small trials suggest L-theanine can lower a person’s blood pressure while they are doing a task that requires attention, and, during times of stress, it may help lower a person’s heart rate and temper spikes of the stress hormone cortisol. There is some evidence, too, that it might boost relaxation and focus.
What effects does L-theanine have on the brain?
Once absorbed into the bloodstream via the gut, L-theanine is able to cross the blood-brain barrier. It has a similar structure to the brain chemicals glutamate and GABA, which are neurotransmitters involved in turning signaling up and down across the brain. Studies in rodents also suggest L-theanine may boost levels of dopamine and serotonin, which are involved in many aspects of brain functioning, including mood and reward.
L-theanine also seems to increase alpha brain wave activity, which is linked to a state of calm, alert wakefulness. “If you close your eyes, alpha goes up all of a sudden,” says Manuel Gomez-Ramirez at the University of Rochester in New York.