L-theanine is found mainly in tea (eg, green tea and black tea), and to a less extent in mushrooms. L-theanine is also available in its purified form as an oral dietary supplement.¹

L-theanine is thought to have both antioxidant and relaxant effects.¹ Some animal studies have shown that the amino acid may also be helpful in lowering lipids in the blood, fighting obesity, and preventing cognitive dysfunction and stroke.¹ L-theanine has been shown to increase the anti-tumor activity of two cancer-fighting drugs, doxorubicin and idarubicin .^2,3

Although there have been a handful of studies evaluating L-theanine for several conditions,¹³ most research has been centered on its effects on mental status.

L-theanine has been shown to have a direct influence on brain activity, such as reducing stress.^4,12 At high doses (higher than usual doses found in a cup of black tea [20 mg (milligrams)]), it has the ability to relax the mind without causing drowsiness. One study measured a type of activity in the brain, called alpha activity, which plays an important role in attention.⁵ Thirty-five participants were given either 50 mg of L-theanine or placebo. Electroencephalogram (EEG) tests were done at baseline and then at specified times afterwards (45, 60, 75, 90, and 105 minutes). Researchers found that there was a greater increase in alpha activity in those who took L-theanine compared to placebo, demonstrating that the amino acid had an effect on the participants’ general state of mental alertness and arousal.

A handful of studies have also looked at the combination of L-theanine and caffeine on various aspects of cognition and mood (eg, alertness, attention).^8,9,10,11 For instance, one study compared the effect of caffeine with and without L-theanine on cognition and mood in 27 participants.¹⁰ Researchers gave participants placebo, caffeine, or caffeine plus L-theanine and measured their performance on several cognitive tests at baseline and then at 60 and 90 minutes after treatment. They found that the combination of caffeine and L-theanine improved speed and accuracy in an attention-switching task, as well as reduced the likelihood of distraction during a memory task. Caffeine alone did improve alertness and accuracy.

Researchers in Korea conducted a randomized, double-blind, placebo-controlled study to look at the effect of L-theanine on memory and attention in 91 people with mild cognitive impairment.⁶ Half of the participants were given 1,680 mg of L-theanine (LGNC-07) and the other half was given placebo for 16 weeks. Memory and word tests were conducted, as well as EEG tests. The researchers found that L-theanine improved memory and selective attention, as well as cognitive alertness.

Another small study also found the amino acid to have a positive effect in reducing anxiety.⁷ The double-blind, randomized, placebo-controlled study was made up of 60 people diagnosed with schizophrenia or schizoaffective disorder. Of the 40 patients who completed the study, L-theanine was associated with reduced anxiety and was found to be well-tolerated and safe.

There have not been any side effects reported from taking L-theanine. However, drinking large amounts of green tea in general may cause upset stomach, irritability, and nausea due to the tea’s caffeine content. Also, do not take L-theanine if undergoing chemotherapy or are taking lipid-lowering medicines or sedatives, since L-theanine may alter the effects of these drugs.