Tests have shown that some brands do not even contain measurable amounts of the substance for which you are buying the product. Supplements with USP on their label have passed certain qualifications for purity and potency of ingredients. It's important to realize that regulation of these supplements is fairly loose. This means that you can't believe everything that the manufacturer says on the label or in advertising literature. If an ad sounds too good to be true, then it probably isn't true.
Natural chemicals are still chemicals.
Products can't be effective and harmless at the same time. Anything that has a physiological effect can be harmful at some dose. One of the strongest reservations expressed by nutrition and fitness professionals regarding the use of supplements is the lack of data on the long-term safety of many products. But many supplements contain potent herbs that are not well understood. Scientists do not yet understand all of the biochemical pathways these hormones affect.