Guarana (Paullinia cupana) is a plant from the Amazon. It is a common ingredient in energy drinks and can be dangerous in large amounts. Guarana contains caffeine. Caffeine works by stimulating the central nervous system, heart and muscles. Guarana also contains theophylline and theobromine, which are chemicals similar to caffeine. People take guarana orally for obesity, athletic performance, mental performance, to increase energy, and for many other reasons.