Evidence Level
Strong
L-Theanine has a robust evidence base, particularly for its synergy with caffeine. Owen et al. (2008) demonstrated that the combination improves attention and task-switching more than either alone. Nobre et al. (2008) used EEG to confirm alpha wave increases within 30 minutes of ingestion. Hidese et al. (2019) found that 200mg/day L-Theanine for 4 weeks reduced stress-related symptoms and improved cognitive function in a randomized placebo-controlled trial. Its excellent safety profile and rapid onset make it one of the most accessible nootropics available.