L-theanine is a naturally occurring amino acid, although it isn’t used as a building block for protein synthesis,^[1][2] unlike the 20 standard protein-forming amino acids that are synthesized by the genetic code. Several types of tea, including black, oolong, and green tea, contain theanine (generally <50 mg per serving).^[3][2][4] When higher doses are desired, theanine can also be taken as a dietary supplement; the theanine in supplements may be extracted from tea^[3] or chemically synthesized using bacterial enzymes.^[5]

The current evidence shows that supplementation with L-theanine may help reduce stress and anxiety in people experiencing acutely stressful situations.^[6] Furthermore, L-theanine may also prevent the increase in blood pressure caused by stressful mental tasks.^[7] Some small studies have also found that L-theanine might lower depression and anxiety scores in healthy people and in people with major depressive disorder.^[8][9]

Supplementation with L-theanine may also improve some aspects of cognitive function, including attention, executive function, and memory.^{[10][11][8][9]} That said, L-theanine has been found to impair executive function and lower attentional control when people are in an emotionally aroused state^[12] or are undertaking stressful mental tasks.^[13]

Further evidence suggests that supplementation with L-theanine might also enhance sleep quality^[8][9] by promoting a more relaxed state in the brain.^[14] However, it is important to note that this evidence is derived predominantly from studies of healthy people. There is currently insufficient evidence to make firm conclusions about whether theanine can improve stress, anxiety, depression, cognitive function, or sleep quality in people with chronic conditions.

The main drawback is that the specific effects of L-theanine have only been tested in a small number of randomized controlled trials which predominantly included healthy people without chronic conditions. Therefore, further high-quality studies are needed to bolster the current evidence.

The clinical evidence cited above shows that supplementation with L-theanine is well-tolerated, with no known reports of adverse effects or toxicity. In rodents, L-theanine has remarkably low toxicity, with one study failing to find any toxic effects when administering up to 4,000 mg of L-theanine per kg of body weight daily for 13 weeks.^[15] Additional animal studies have reported similar results with no observed toxicity at remarkably high doses.^[1] Consequently, it is generally assumed that L-theanine is safe for humans to consume,^[1] particularly given its long history of consumption in tea. However, specific safety studies in humans are lacking.

After consumption, L-theanine can cross the blood-brain barrier^[16] and affect brain activity by promoting increased alpha-wave activity,^[17][1][2] a pattern of brain activity associated with a more relaxed state. This may explain its stress-reducing and anxiety-reducing effects.^[8]

In animal studies, L-theanine has been shown to affect neurotransmitter signaling in the brain by behaving like a glutamate reuptake inhibitor as well as a glutamate receptor antagonist.^[18] Other animal studies demonstrate that L-theanine can also exert neuroprotective effects via its action on gamma-aminobutyric-acid-A (GABA-A) receptors.^[19] However, further investigation in humans is needed to confirm these mechanisms of action.^[1][2]