After consumption, L-theanine can cross the blood-brain barrier[1] and affect brain activity by promoting increased alpha-wave activity,[2][3][4] a pattern of brain activity associated with a more relaxed state. This may explain its stress-reducing and anxiety-reducing effects.[5]
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.[6] Other animal studies demonstrate that L-theanine can also exert neuroprotective effects via its action on gamma-aminobutyric-acid-A (GABA-A) receptors.[7] However, further investigation in humans is needed to confirm these mechanisms of action.[3][4]