Guarana is a South American shrub traditionally used by Indians to help maintain energy levels. Today, Brazilians use guarana as a health tonic.

Guarana seeds contain 4 percent to 8 percent caffeine, as well as trace amounts of theophylline and theobromine. These chemicals are believed to account for guarana's energy-stimulating and fat-burning effects (Carlson et al 1998).

Toxicology studies assessing guarana's effect in mice and rats demonstrated high doses of guarana (1000 to 2000 mg/kg) had no significant toxicity effects while low doses (1.2 mcg/mL) actually had an antioxidant effect (Mattei et al 1998).

In animals, guarana has been shown to increase physical endurance under stressful conditions to a greater extent than do comparable doses of caffeine or ginseng (Espinola et al 1997).

In another study, Guarana extract in an herbal formulation given to overweight human patients for 45 days was associated with an 11.2-lb weight loss in the guarana group compared with less than a 1-lb weight loss in the placebo group (Andersen et al 2001).

Guarana is well tolerated. However, adverse side effects, including heart palpitations and anxiety, have been reported when guarana is combined with powerful CNS stimulants such as ephedra and bitter orange (Citrus aurantium) (Pittler et al 2005).

Therefore, guarana should not be taken in conjunction with CNS stimulants.