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Objective:  L-theanine, a non-proteinic amino acid found in tea, is known to enhance attention particularly in high doses, with no reported adverse effects. We aimed to determine whether oral administration of L-theanine acutely enhances neurophysiological measures of selective attention in a dose-dependent manner. Methods:  In a double-blind, placebo-controlled, counterbalanced, 4-way crossover study in a group of 27 healthy young adults, we compared the effects of 3 doses of L-theanine (100, 200 and 400 mg) with a placebo (distilled water) on latencies of amplitudes of attentive and pre-attentive cognitive event-related potentials (ERPs) recorded in an auditory stimulus discrimination task, before and 50 min after dosing. Results:  Compared to the placebo, 400 mg of theanine showed a significant reduction in the latency of the parietal P3b ERP component ( p  &lt; 0.05), whereas no significant changes were observed with lower doses. A subsequent exploratory regression showed that each 100-mg increase in dose reduces the P3b latency by 4 ms ( p  &lt; 0.05). No dose–response effect was observed in P3b amplitude, pre-attentive ERP components or reaction time. Discussion:  The findings indicate L-theanine can increase attentional processing of auditory information in a dose-dependent manner. The linear dose–response attentional effects we observed warrant further studies with higher doses of L-theanine.

L-theanine improves neurophysiological measures of attention in a dose-dependent manner: a double-blind, placebo-controlled, crossover study