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If you think you’re doing your body good by popping a daily multivitamin or supplementing with extra vitamin C or E, think again. A new study finds that taking supplements is linked to higher odds of early death, at least in older women.

Jaakko Mursu, a nutritional epidemiologist at the University of Minnesota, reports with his colleagues in the Archives of Internal Medicine that women who took multivitamins were 6% more likely to die over a 19-year period, compared with women not taking them.

That news will come as a surprise to many of us, although scientists have suspected for some time that vitamins and supplements may not be as beneficial to health as previously believed. In recent years, studies have shown that vitamins such as A, C and E, which were supposed to lower risk of chronic illnesses like heart disease and cancer, didn’t provide much benefit. But many patients kept taking them anyway, and few doctors actively discouraged it, since the studies didn’t show that taking vitamins did much harm either.

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But Mursu and his team found that using multivitamins, which nearly half of all American adults do, was linked to a higher risk of death among a group of 38,000 women, average age 62, who were studied for nearly two decades. The researchers also looked at a variety of other supplements and found higher odds of death associated with six of them:

Vitamin B: 10% higher risk of death, compared with nonusers

Folic acid: 15%

Iron: 10%

Magnesium: 8%

Zinc: 8%

Copper: 45%

The one bright spot: calcium, which many older women take to protect against osteoporosis and bone fractures. Women in the study who took calcium had a nearly 10% lower risk of death over the study’s follow-up period, compared with those who didn’t take calcium supplements.

“Looking at our findings, and combined with previous studies, the overall main message is that there are no benefits to taking multivitamins or supplements, at least if the hope is to prolong life or prevent disease or cancer,” says Mursu.

In fact, Mursu’s study is among the first to quantify the harm that might come from taking vitamins and supplements. Over the course of the nearly 20-year study, researchers surveyed the women three times about whether they took multivitamins or 15 other types of supplements, and in what doses. Over time, the researchers found that the proportion of women taking supplements went up: from 65% in 1986 to 75% in 1987, then up to 85% by 2004.

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Interestingly, at the start of the study, the women who reported using vitamins and supplements were healthier overall than nonusers — they were less likely to be overweight and have diabetes or high blood pressure and were more likely to eat more fruits and vegetables and avoid high-fat foods. Such healthy profiles are usually linked to a lower risk of early death, but in this group, that wasn’t the case. And the reason, says Mursu, was likely due to the supplements.

Even after he and his team adjusted for the effect of lifestyle factors such as weight, smoking, blood pressure, diet and physical activity, the relationship between supplement use and higher death risk remained, suggesting the correlation was real.

So why are supplements, which are supposed to improve health, linked to such harm? Mursu thinks that in some cases, women may be taking too much of a supplement, making it toxic rather than therapeutic. In 2004, studies showed that 27% of women used four or more supplements daily, and most of these included a multivitamin, which in itself contains the recommended daily allowances for nutrients. “Most supplements contain high amounts of specific compounds, and high doses could be toxic,” says Mursu. “If you combine several supplements, or a multivitamin with supplements, then you reach even higher potentially toxic doses.”

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Which means that if you’re not taking supplements to treat a specific condition — like taking iron to combat anemia — but are only hoping to live longer and healthier, you might want to swap out your vitamins for a healthier diet instead. “I would advise women to reconsider whether they need to use supplements,” says Mursu. “I would vote for improving their diet instead. It’s safer and more practical.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.