It’s important to know that herbal supplements in general are developed and marketed in the United States as food stuffs. This means they don’t require clinical studies to test their efficacy or to measure their side effects.
Many manufacturers employ rigorous quality-control measures and produce high quality products, whereas others produce poorer quality products with regard to purity and levels of active compounds.
Choose brands labeled with the NSF International, US Pharmacopeia, Underwriters Laboratory, or Consumer Lab seal. These verify that the product actually contains the ingredients that the label says it does, and that the product doesn’t have any potentially harmful ingredients. Xymogen and Metagenics are two such brands.