This systematic research has never been done for dietary supplements. Although most will be overlooked, some deaths and serious injuries will nevertheless be discovered. The reporting system is entirely voluntary, even for manufacturers. With the much better known FDA MedWatch system for prescription drugs, only about 1 in 100 serious adverse events are reported. For example, when Burton cited the estimate of 106,000 annual deaths from prescription drugs, he did not mention that only about 1,500 of these are reported to the FDA and less than 200 appear on death certificates.
Nobody really knows.
In theory, they ought to be safer, significantly safer. Industry advocates and their allies in the alternative health community are fond of saying that the FDA has plenty of legal authority to remove those dietary supplements that are dangerous. The legal authority may be there, but a working system is not. Without systematic safety testing, there is no scientific information base to provide the warning flags, the clues where to look for problems. And once reported, a staff of only three has the job of investigating.