"People think herbs are harmless because they are considered 'natural,' and they think, if one is good, 10 are better," said Dr. Shapira. But overdoing it with supposedly safe herbs can cause health problems as serious as internal bleeding and heart arrhythmia, Dr. Shapira warns.

He also points out that all drugs are natural. "Almost all medicines start out as plants, but established drugs have the benefit of being standardized and regulated by the Food and Drug Administration," he says. Herbal medications are not standardized or regulated in any way.