USP just backed up Dr. Offit’s claim that a mere pittance of dietary supplements are USP-verified. Hardly any meet standards set by the U.S. Pharmacopeia for ensuring that what is on a supplement’s label is what is in the bottle:

@fanaticcook although we do not have an exact figure, historically it's been less than 1%.

— U.S. Pharmacopeia (@USPharmacopeia) February 24, 2014

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USP ensures ensure the strength, quality, purity, and believe it or not, the identity of a product. You would think that a bottle of, say, St. John’s wort contains St. John’s wort, but:

“One bottle of St. John’s wort was wholly made of ground rice, while another contained a known laxative called Alexandrian senna. Neither of the 2 bottles tested contained the common active ingredient in St. John’s wort.”

I guess companies really can get away with selling cheap fillers in lieu of active ingredients.