While research into how herbs work continues, we unfortunately still have a very limited understanding of the complex mechanisms underlying many botanical substances.
The FDA, which must approve the safety and effectiveness of conventional drugs before they can be sold, does not regulate herbal medicines. The responsibility to ensure the safety of products falls on of the manufacturer alone—but manufacturers often do not provide consumers with information about the potential adverse side effects or problematic interactions that might be caused by taking an herbal supplement along with other conventional drugs. Often there are no warning labels to alert people with certain conditions that their illness might be exacerbated by the herbal remedy.
It is very important to discuss taking an herbal medicine with your doctor before beginning any supplement. Remember that herbs are medicines and need to be taken with the same precautions as you'd use with any pharmaceutical drug.