Caffeine + L-Theanine Stack

In a crossover study of 27 healthy volunteers, a single 100 mg L-theanine plus 50 mg caffeine combination improved speed and accuracy on an attention-switching task at 60 minutes, and reduced susceptibility to distracting information at 60 and 90 minutes after dosing, beyond the effects of 50 mg caffeine alone. This does not establish durable cognitive enhancement or elimination of caffeine's adverse effects.^[1]

• Small acute study, multiple tasks and time points; no long-term clinical outcomes.
• This single 27-participant trial is one of 16 trials pooled in a 2025 systematic review and meta-analysis (see the companion systematic-review finding for this compound); it should be read alongside that broader, more uncertain evidence base rather than in isolation.
• Abstract-checked; no qualified clinical review.

A 2025 systematic review and meta-analysis (PROSPERO-registered) pooled 16 randomized trials (484 participants in the meta-analyzed subset, including the 27-participant Owen et al. 2008 trial cited elsewhere for this compound) testing tea, L-theanine, or L-theanine plus caffeine against placebo in healthy people. Combined L-theanine plus caffeine showed small-to-moderate favorable point estimates on measures such as digit-vigilance accuracy and attention-switching accuracy, but confidence intervals for several outcomes, including choice reaction time and overall mood, crossed zero and did not reach statistical significance. L-theanine alone showed a small but statistically significant advantage over placebo on choice reaction time in the first hour.^[2]

• Industry-funded or industry-affiliated for a majority of the underlying trials (industry was the most common funding source among the 16 meta-analyzed trials) and for this review itself (two of five authors are employees of tea/tea-ingredient companies); this is a material funding limitation across the evidence base.
• High between-study heterogeneity (I-squared values up to about 96%) and small per-outcome trial counts limit precision; several point estimates favored the active treatment without excluding no effect, and some analyses favored placebo.
• This synthesis characterizes the wider evidence base and does not itself re-measure or replace the original Owen et al. 2008 finding cited in this compound's primary combination-outcome claim.
• Passage-checked in full text (PMC Open Access, CC BY-NC-ND); no qualified clinical review.

A controlled study found cardiovascular responses with oral ephedrine plus caffeine. The site records also include aspirin and L-theanine, which were not studied; the selected product combination is therefore an extrapolation.^[3]

• Single-dose study in 16 healthy adults; no evidence that every stimulant pair is synergistic.