context: Guarana is derived from the seeds of a South American tree. Because it's high in caffeine, guarana has become a popular energy supplement. Researchers once believed that the active ingredient of guarana was a chemical specific to the plant -- guaranine. But they later discovered that it was just caffeine. Guarana has among the highest concentrations of caffeine in any plant. It may contain up to 3.6% to 5.8% caffeine by weight. Coffee only has up to 2%. Coffee and other caffeinated beverages seem to increase alertness when consumed throughout the day. Caffeine can improve mental performance and alertness in sleep-deprived people, and guarana would logically have the same effect. Experts have not studied guarana extensively. So far, studies have not found that guarana is helpful for improving cognitive ability or mood. Some people use guarana to boost athletic or sexual performance. But there's no evidence to back this up. There's some evidence that guarana â when used along with other supplements â may promote weight loss. It's uncertain if the guarana specifically was responsible. Because caffeine is a stimulant that has been shown to improve mood and aid weight loss, guarana may very well also have these effects. As a weight loss supplement, caffeine may work best when combined with the polyphenol compounds in green tea. Combining guarana with polyphenols may have a similar effect. The safety of using guarana for weight loss is unknown. Some believe that the effects of guarana are less intense and longer-lasting than caffeine. Studies have not established this. There is no standard dose of guarana. Ask your doctor for advice. Besides the guarana seed itself, there are no natural food sources of guarana. It has become a common additive to some foods and drinks. Side effects. The side effects of guarana are generally the same as the side effects of caffeine. They include sleep problems, anxiety, restlessness, upset stomach, and quickened heartbeat. Long-term use of caffeine may result in tolerance and psychological dependence. Risks. High doses of caffeine may raise the risk of increased heart rate, breast disease, and high blood pressure. An overdose of caffeine can cause seizures and convulsions. Talk to your doctor before using guarana if you have high blood pressure, anxiety disorders, glaucoma, osteoporosis, heart problems, bleeding disorders, diabetes, kidney problems, or liver disease. At least one death has been reported in a young woman with mitral valve prolapse who consumed a guarana containing drink. People with MVP should speak to their doctor before using guarana. Interactions. If you take any medications regularly, talk to your doctor before you start using guarana supplements. They could interact with medicines like some antidepressants, lithium, sedatives, and blood thinners. Don't use guarana along with other stimulants, whether they're medications, supplements, or street drugs. To avoid excessive caffeine intake, be careful when taking guarana along with other foods and supplements that contain caffeine, such as coffee and sodas. Given the lack of evidence about its safety -- and because of its high caffeine content -- guarana is not recommended for children or for women who are pregnant or breastfeeding.