There is no need for most people to take vitamin supplements and some may even be linked to a higher risk of dying in older women, according to a study.
Iron stood out among supplements as a particular concern, while calcium appeared to be linked to lower death risk, said the study in the Archives of Internal Medicine, a journal of the American Medical Association.
With about half of Americans taking vitamin pills of some kind, the study aimed to examine whether the US$20 billion (NZ$25.1bn) supplement industry was having any effect on extending lifespan in an already well-nourished population.
The researchers confirmed their theory - that supplements were not helping people ward off death. But the reasons for the link to higher risk of overall mortality, or the risk of dying for any reason, were less clear.
"Based on existing evidence, we see little justification for the general and widespread use of dietary supplements," wrote the study authors from the University of Eastern Finland and the University of Minnesota.
"We found that several commonly used dietary vitamin and mineral supplements, including multivitamins, vitamins B6, and folic acid, as well as minerals iron, magnesium, zinc, and copper, were associated with a higher risk of total mortality."
The US and Finnish researchers examined data from the Iowa Women's Health Study, including surveys filled out by 38,772 women with an average age of 62.
Women self-reported their supplement use in 1986, 1997 and 2004, and data showed their use rose from 66 per cent of survey-takers at the start to 85 per cent by 2004.
Those who took supplements showed a range of healthy lifestyle factors, and were more likely than non-supplement users to be non-smokers, eat low-fat diets and exercise.
But in many cases they showed a higher risk of dying than their supplement-free counterparts.
"Of particular concern, supplemental iron was strongly and dose dependently associated with increased total mortality risk," said the study.
On the other hand, "supplemental calcium was consistently inversely related to total mortality rate", meaning that calcium-takers showed a lower death risk, though the same dosage relationship was not visible.
The authors said they could not rule out the possibility that the reason for the higher death rate in iron users could have been due to underlying conditions for which they were taking supplements, and more research is needed.
In the meantime, doctors urged patients to consider the risks of taking supplements unless they are needed to stave off deficiencies.
"We think the paradigm 'the more the better' is wrong," wrote doctors Goran Bjelakovic of the University of Nis in Serbia and Christian Gluud of Copenhagen University Hospital in Denmark in an accompanying commentary.
These findings "add to the growing evidence demonstrating that certain antioxidant supplements, such as vitamin E, vitamin A, and beta-carotene, can be harmful," they said.
"We cannot recommend the use of vitamin and mineral supplements as a preventive measure, at least not in a well-nourished population."
Bjelakovic and Gluud said the only supplement that may be beneficial to older women, and possibly older men, is vitamin D3, if they do not already get enough through their diet or from sun exposure.
"The issue of whether to use calcium supplements may require further study," they added.