Supplements are drugs! But since they are unregulated, you can’t be sure you are getting what you think! Look for supplements that carry the seal of good manufacturing process, which indicates the brand follows voluntary quality standards. As with other drugs. Check for food-drug interactions of all prescription and over the counter medications and supplements.
Here is a piece from the NY Times, informing the public about the potential dangers:
Dietary supplements: mislabeled, contaminated and probably useless