What’s in your vitamin supplement? It could be more or less than you think, according to the latest study to show that what’s on a supplement label is not necessarily what’s in the bottle.

Researchers who tested vitamin D pills sold in stores found they contained from 9% to 140% of the doses listed on labels, according to a research letter published Monday in JAMA Internal Medicine. Though none of the pills was likely to be dangerous, some contained too little of the vitamin to effectively treat someone with a deficiency, the researchers say.

We found the potency of these vitamin D supplements varied widely

says Erin LeBlanc, an endocrinologist who led the study at the Kaiser Permanente Center for Health Research in Portland, Ore.

Other studies have found similar variations in other supplements. For example, more than one-third of multivitamins tested by ConsumerLab.com in 2009 were contaminated or contained significantly more or less of some ingredients than their labels claimed. The company also recently tested vitamin D samples and found some mislabeling, it says.

About half of adults in the USA take dietary supplements, spending $30 billion a year, according the Office of Dietary Supplements, National Institutes of Health. Vitamin D is particularly popular. The vitamin is essential for bone health and may have other benefits, but most people get enough from food and sun exposure, according to the Institute of Medicine.

LeBlanc decided to test over-the-counter vitamin D pills after testing pills made at a compounding pharmacy for another study. She was surprised to find that those specially made pills contained 52% to 146% of the promised vitamin D doses.

“I was curious about what else was out there,” LeBlanc says. She and her team went to five “mainstream grocery stories” in Portland, she says, and bought 55 bottles of vitamin D supplements from 12 different manufacturers. Promised potencies ranged from 1,000 to 10,000 international units (IUs).

The actual amounts in the pills ranged from 9% to 140% of what was on the label. When researchers tested five pills from each bottle and averaged the results, levels were closer to 100%, but in a third of cases, they were still too high or low by the standards set by one independent testing group, the U.S. Pharmacopeial Convention (USP). The closest match was in the one brand with a USP mark, the researchers say.

“It’s not surprising that they found a lot of products that were not meeting label claims,” says John Atwater, director of the verification program at the non-profit USP, one of several groups that offer voluntary testing programs and approval seals. Supplement makers must pay to participate in those programs.

Unlike drug companies, supplement makers are not required to prove their products are safe and effective before selling them. The Food and Drug Administration can inspect supplement plants for good manufacturing practices and has issued warnings to many of them under regulations finalized in 2007. But the FDA does not test every product, even at plants under inspection, Atwater says.

Companies that put out mislabeled products “are breaking the law,” says Duffy MacKay, vice president of scientific and regulatory affairs for the Council for Responsible Nutrition, a supplement trade association. He says responsible companies are responding to FDA and public pressure to do better. He suggests consumers look for well-known brands and voluntary testing seals.

“We’re hoping that soon we’ll have a perfect record,” MacKay say. “We advocate very strongly for 100% compliance.”

LeBlanc says her study is evidence that more regulation is needed to get companies in line.