Guarana is a plant. It is named for the Guarani tribe in the Amazon, who used its seeds to brew a drink. Today, guarana seeds are still used as medicine.
People take guarana for weight loss, athletic performance, mental performance, to increase energy, as an aphrodisiac, and for many other conditions, but there is no good scientific evidence to support these uses. Guarana can also be unsafe when taken long-term in large amounts.