The amount of vitamin D in some supplements may be either much lower or much higher than what’s written on the label, according to a new analysis.

Researchers found that off-the-shelf pills from 12 different manufacturers had between 52 percent and 135 percent of their advertised vitamin D content.

And among vitamins mixed by compounding pharmacies, the variation in doses was even greater – from 23 percent to 146 percent of the labeled amount.

“I’m not at all surprised that they’re very variable,” said Dr. Pieter Cohen, who studies dietary supplements at Harvard Medical School in Boston but wasn’t involved in the new research.

“When you need a supplement to work, it’s really hard to find one that does,” he added – in part because of lax regulation.

Vitamin D supplements can be bought for a few dollars per month.

Together with calcium, they have been tied to improved bone health. Other medical claims made for extra vitamin D – such as its ability to lower blood pressure or boost immunity – are more tenuous.

For the new study, Dr. Erin LeBlanc from the Kaiser Permanente Center for Health Research in Portland, Oregon, and her colleagues chemically analyzed pills from 15 vitamin D bottles purchased at local stores and two doses of compounded vitamins.

Supplement bottles were labeled as containing 1,000, 5,000 or 10,000 international units (IU) of vitamin D. Just one-fourth of the vitamins met the standard of all pills falling between 90 and 120 percent of the expected dose, based on a random selection of five pills per bottle.

Pills made by the one manufacturer that was verified by the U.S. Pharmacopeial Convention (USP) – a third-party tester – were all within six percent of the listed dose, LeBlanc and her colleagues found.

“Consumers buying those products can be more assured that what they’re getting in their pills is what’s labeled,” she said.