Guarana

Guarana is a plant whose seeds are unusually rich in caffeine, so most of what it does is simply caffeine's well-established effects on alertness, perceived effort and performance. It also contains minor amounts of related compounds and antioxidants.

Because the active ingredient is caffeine, the same evidence, dose sense and cautions apply: effects build over 30–60 minutes, tolerance develops, and late-day use disrupts sleep. Judge and dose it as you would any caffeine source.

Sources

• PubMed: guarana caffeine cognition exercise performance (research overview)