More than half of American adults take vitamin pills. I've watched in wonder as some of my more health-conscious friends kick off their morning with an impressive array of multicolored supplements: A, C, D, calcium, magnesium, you name it. And it's not just my friends: data from the National Health and Nutrition Examination Survey (NHANES) indicate a trend away from all-in-one multivitamins and toward specific supplements—especially fish oil and vitamin D. Most of this is self-prescribed. According to a 2016 analysis of the NHANES data, less than a quarter of supplements are taken at the recommendation of a health professional.

Most of this nutritional enthusiasm does no harm—apart from the budgetary kind—and for those with inadequate diets or special health concerns, supplements can do a world of good. But it is wise to keep in mind that doses that far exceed the recommended dietary allowances (RDAs) set by the Institute of Medicine can be hazardous. A reminder comes from a recent study linking excessive B vitamins to a heightened risk of hip fracture.

The study, published in May, combed through the vitamin habits of nearly 76,000 postmenopausal women participating in the decades-long Nurses' Health Study. Lo and behold, those who took high doses of vitamin B6 (35 milligrams or more daily), together with B12 (20 micrograms or more), had a nearly 50 percent greater risk of fracturing their hip than those taking low doses or none. High doses of B6 alone also raised the risk. The study confirms similar findings in a large Norwegian trial, published in 2017, that looked at whether these vitamins and folic acid could reduce heart attacks and strokes in patients with narrowed blood vessels. Alas, they did not, and to the great surprise of researchers, high doses were linked to hip fractures. Taken together, “the results are quite convincing,” says Haakon Meyer, a professor of epidemiology and preventive medicine at the University of Oslo and an author of both studies.

Why these vitamins would have such an effect is not clear. Meyer suggests two possible pathways. Too much B6 can be toxic to the nervous system, raising the chances of falling and cracking a hip. The nurses on high doses took 20 to 30 times the RDA, he notes. “Traditionally, we thought the doses needed to get these adverse effects would be much higher, but we don't know for sure.” Another possibility is that B6 competes with estrogen in binding to steroid receptors, compromising the hormone's role in bone health. Both ideas, he says, would require more evidence.

The B vitamin findings are reminiscent of a discovery made some 20 years ago that linked excessive vitamin A (retinol) with hip fractures. This research, published in 2002, also relied on the trove of data from the Nurses' Health Study. Walter Willett, a professor of nutrition and epidemiology at the Harvard T. H. Chan School of Public Health, cites the response from the vitamin industry as “a situation where things worked right.” Manufacturers quickly reduced the amount of vitamin A in their multivitamins, he says, “and without most people being aware of it, a huge number of hip fractures were prevented.” Willett, who is a co- author with Meyer of the new study, suggests that something similar may be in order for B6 and possibly B12 and that an expert panel review would be a sensible next step. “We might deal with most of the problem just by bringing down the level of B6 [in supplements] to the RDA level,” he explains. B12 is a more complex matter, however. Ten to 30 percent of adults older than age 50 need extra B12 because of poor absorption. Meyer points out that excess B12 alone does not seem to raise the risk of fracture.

The bottom line is that although vitamins and minerals are essential for health, more is not necessarily better. Research shows, for example, that taking large amounts of beta carotene (a vitamin A precursor) seems to accelerate lung cancer in smokers, even though the nutrient may have anticancer properties in other contexts. Like everything in nutrition, vitamins are complicated. Just consider the fact that B6 plays a role in more than 100 different enzyme reactions. Perhaps because of that complexity, many seemingly logical uses of vitamins yield disappointing results. For instance, even though low blood levels of vitamin D correlate with greater risk of heart attacks and strokes, taking D supplements generally does not help, according to a 2019 analysis.

Vitamins are vital when your diet is deficient. Willett thinks a daily multivitamin is a sensible insurance policy. The irony, he observes, is that the people most apt to take lots of supplements are educated folks with a healthy diet—in other words, those who need them the least.