Beware of dietary supplements that contain ephedra or guarana. Even taking them as directed can harm your heart and metabolism, according to a recent study. Plus, they may be especially dangerous for people with cardiovascular problems or high blood sugar. Guarana contains caffeine. A few years ago, ephedra was banned as a dietary supplement ingredient by the U.S. Food and Drug Administration. However, it still is available as “ma huang,” a traditional Chinese medicine.

- Clinical Pharmacology and Therapeutics