It depends on the herbs. Most herbs sold as dietary supplements are very safe. When used appropriately, the majority of herbs used by practitioners have no adverse side effects. A review of the traditional and scientific literature worldwide demonstrates that serious side effects from the use of herbal medicines are rare. According to Norman Farnsworth: “Based on published reports, side effects or toxic reactions associated with herbal medicines in any form are rare. In fact, of all classes of substances reported to cause toxicities of sufficient magnitude to be reported in the United States, plants are the least problematic.”