What it's actually good for
L-theanine is an amino acid found naturally in tea leaves (Camellia sinensis) that crosses the blood-brain barrier and promotes a state of calm alertness. Unlike most anti-anxiety compounds, it does not cause sedation or cognitive impairment at standard doses. The evidence base includes several well-designed RCTs showing that 200 mg of L-theanine increases alpha brain wave activity (associated with relaxed focus), reduces physiological and subjective stress responses, and modestly improves attention. The strongest and most consistent data is for the combination of L-theanine with caffeine, where the two appear to synergize: caffeine provides alertness while L-theanine smooths out the jitteriness. On its own, effects on cognition are real but modest. It earns a B because while the trial data is consistent and the mechanism is well-characterized, most individual studies are small and the cognitive improvements, though statistically significant, are not dramatic.