The Journal of the American Medical Association recently published a study done by scientists in Cambridge, Mass.  This group did refined chemical analysis on 60 different health supplements.  Their findings were very disconcerting.  Just 11 percent of the products contained an accurate amount of the key ingredients listed on the label.  Forty percent had none of the indicated ingredients.  Forty Five percent had inaccurate amounts varying from .02 percent of the amount listed to 334 percent of the amount listed. Finally, 8 percent of the products contained at least one compound prohibited by the US FDA. 

As opposed to prescription medicines, the FDA does not have authority to approve supplements before they are marketed.  The agency does require that OTC products contain what they indicate. 

Just because a product is on the market does not mean it’s safe, much less effective.  This is probably where an ounce of skepticism is worth a pound of cure.