Biotin, promoter of "Cher hair" and Rihanna-worthy nails, has always been a little controversial since claims that the vitamin can help you grow Rapunzel-length strands have never been substantiated by the FDA. Now, a new FDA warning outlines another caveat to consider before taking biotin: It could mess with blood test results.

Last year, we reported on a study suggesting that taking high doses of biotin could skew certain diagnostic test results leading to false positives or false negatives for everything from pregnancy to cancer. In a statement released this week, the FDA has confirmed the concern. "The FDA is alerting the public, health care providers, lab personnel, and lab test developers that biotin can significantly interfere with certain lab tests and cause incorrect test results which may go undetected," the statement reads.

Here's the deal: Biotin in your body (whether it comes from your diet or from popping a pill) bonds with certain proteins, which can be measured to detect a variety of health conditions. Doctors can measure your biotin levels as a way to determine the presence of said protein markers, but if you're taking a biotin supplement, that can make the test results inaccurate.

These falsely skewed tests could lead to serious consequences. According to the statement, "The FDA has seen an increase in the number of reported adverse events, including one death, related to biotin interference with lab tests."

Biotin is a natural vitamin — we get it from food like eggs, whole grains, and almonds, so in the right amounts, it's not a problem. The risk of ruining important test results comes into play with supplements, which can have up to 650 times the recommended daily intake of the vitamin, says the FDA. It’s this level of biotin that could cause a false positive on a pregnancy test or a false negative in a cancer screening.

To avoid a potentially dangerous test result, make sure your doctor knows if you're taking biotin (remember, it might be in your multivitamin or prenatal vitamin even if you're not taking a specific supplement, adds the FDA) and that any lab you go to for blood work knows as well. "It's important to discuss any supplements (over-the-counter, vitamins, minerals, etc.) you are taking with your doctor because these supplements can have potentially adverse effects," says Sejal Shah, a dermatologist and founder of SmarterSkin Dermatology in New York City. "Just because something is OTC or natural doesn't mean that it is completely harmless."

If you think biotin might have played a role in the results of a recent lab test, report it to the FDA. "If you suspect or experience a problem with a laboratory test while taking biotin, we encourage you to file a voluntary report through MedWatch, the FDA Safety Information and Adverse Event Reporting program," the agency says.

The bottom line: Let your health-care team know you're popping extra biotin. Your doc will advise you what to do if you need biotin-based blood work.

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