One of the problems with buying vitamins and supplements is that the Food and Drug Administration does not regulate them the way they regulate prescription drugs. As a result, you may not be getting the potency you expect.

Researchers analyzed 55 different bottles of vitamin D. The pills came from the same manufacturer, but were from different lots. They found potencies as low as 9 percent and as high as 140 percent of the listed dose.

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Although supplement manufacturers don’t have to conform to FDA standards, they can comply with the standards set by the U.S. Pharmacopeial Convention (USP), which helps ensure quality.

When buying your supplements, look for the USP stamp on the bottle.