http://www.nejm.org/doi/full/10.1056/NEJMp0904768
Before 1994, herbal products were considered food additives, and their manufacturers were required to show proof of safety before marketing them. Since the passage of the DSHEA, dietary supplements are presumed to be safe and can be marketed with very little oversight.
The DSHEA presents serious obstacles to the FDA’s ability to detect and eliminate contaminated supplements. A wide range of dietary supplements have been found to be contaminated with toxic plant material, heavy metals, or bacteria. Of particular concern are the dozens of dietary supplements that are contaminated with prescription medications, controlled substances, experimental compounds, or drugs rejected by the FDA because of safety concerns.
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Always research your supplements with fervor!