Study leader Erin LeBlanc, an endocrinologist at the Kaiser Permanente Center for Health Research in Portland, Ore., said none of the supplements was likely to be dangerous, but some contained too little of the vitamin to effectively treat someone with a deficiency, USA Today reported.
LeBlanc and her team went to five "mainstream grocery stories" in Portland and bought 55 bottles of vitamin D supplements from 12 different manufacturers ranging from 1,000 to 10,000 international units.
Although actual amounts of vitamin ranged from 9 percent to 140 percent, when researchers tested five pills from each bottle and averaged the results, levels were closer to 10 percent.
However, in a third of the cases, the vitamin D was still too high or low by the standards set by one independent testing group, the U.S. Pharmacopeial Convention, the study said.
The findings were published in Internal Medicine.