Li and her colleagues demonstrated this problem with an experiment. They asked six healthy adults to take 10 milligrams of biotin as a dietary supplement for a week. They tested their blood before and after they took the supplements for nine different hormones, a cancer marker, and iron. About 40% of the tests were thrown off by the supplements. Their study was published in September.

“For the last couple of years, I have been trying to raise the profile of this issue and get it to the attention of people who might be able to fix it. I’m glad to see this [FDA warning]” says Earle Holmes, PhD, a professor of molecular pharmacology and therapeutics at Loyola University School of Medicine in Maywood, IL.

Holmes and his team reviewed 374 tests that run on some of the most popular lab testing machines in the U.S. He found that 221 of them use biotin in their test. About 80 of these tests came with instructions that indicated they could be skewed by extra biotin in a patient’s blood.

The FDA says it is “working with stakeholders to better understand biotin interference with laboratory tests, and to develop additional future recommendations for safe testing in patients who have taken high levels of biotin when using laboratory tests that use biotin technology.”

To stay safe, Li says patients should check their vitamin bottles for biotin. Learn how much you’re taking, and tell your doctor you’re taking it at your next visit. She says lab directors can sometimes change tests if they think biotin might be a problem.

“This is a message that needs to get out,” says Lynn Burmeister, MD, an endocrinologist and associate professor of medicine at the University of Minnesota.

“If I can control what my patients are doing, I tell them to stop biotin for 1 week,” Burmeister says.

Other doctors say it’s probably also wise to consider whether you need to be taking biotin at all. Foods containing the vitamin include liver, egg yolks, fish, meat, seeds, nuts, and some vegetables, such as sweet potatoes.

“Biotin is in many supplements sold to improve skin and hair; however, the science supporting biotin for these indications is quite weak, so I don’t recommend it to any of my patients,” says Pieter Cohen, MD, a researcher at Harvard and associate professor of medicine at the Cambridge Health Alliance in Somerville, MA.