Women taking multivitamins live no longer than those who don't take the pills, and might actually die sooner, a huge new study has found.

The study involved about 39,000 women who were between the ages of 55 and 69 when the study began and were tracked for 19 years.

During that time, about 40 per cent of them died. When the researchers looked at who took vitamins, those who chose multivitamins had a slightly higher risk of death than those who took no supplements at all. The same was found for women who regularly took iron, vitamin B6, folic acid, magnesium, zinc and copper.

In fact, of the 15 supplements the researchers looked at, only calcium was associated with a lower risk of death.

While the study, which appears in the Archives of Internal Medicine, didn't look at men, the authors say they expect the findings would likely be the same for them as well.

The study only found a link between vitamins and increased risk of death; it didn't prove that the supplements caused the deaths. The authors say the reasons behind their findings aren't clear.

"We saw an increased risk of total mortality, but we don't really know what is the reason," lead author Jaakko Mursu, of the University of Eastern Finland and the University of Minnesota told CTV News.

But they note the study adds to the growing body of research that has emerged in recent years that suggests that vitamins might not help to prevent disease, and perhaps, might actually increase the risk of death.

And yet vitamins remain hugely popular, even though little hard research exists on some of the supplements.

In a commentary that goes alongside the study, Dr. Goran Bjelakovic of the University of Nis in Serbia, and Dr. Christian Gluud of Copenhagen University in Denmark say many people continue to have the mistaken notion that if a little bit of vitamin supplementation is good, then more must be better.

"Until recently, the available data regarding the adverse effects of dietary supplements has been limited and grossly underreported. We think the paradigm 'the more the better' is wrong," they write.

"We cannot recommend the use of vitamin and mineral supplements as a preventive measure, at least not in a well-nourished population," they conclude.

Nevertheless, they add that older women, and perhaps men, might benefit from vitamin D supplements, especially if they have insufficient vitamin D from their diet and sun exposure. As for calcium, that "may require further study," they add.

To conduct the study, Mursu led a team who analyzed data from the Iowa Women's Health Study. The women in that study filled out 16-page questionnaires in 1986 about their diet, their use of vitamins and their general health. They recorded their of use of multivitamins, vitamins A, C, D and E as well as beta-carotene, B vitamins and minerals such as calcium, copper, magnesium, selenium and zinc.

They were asked again about their supplement use in 1997 and 2004. The proportion of women who said they took one or more vitamin supplement soared during the study period, from 62.7 per cent in 1986 to 85.1 per cent in 2004.

The researchers found that over the 19 years of follow-up, 41 per cent of multivitamin users died, compared to 40 per cent of non-users.

Taking iron was particularly problematic: the more iron the women took, the higher their risk of death.

Only calcium supplements were linked to a lower risk of death, with 37 per cent of users dying compared to 43 per cent of non-users.

The researchers note that previous research has shown that vitamin users tend to have healthier lifestyles, in general. But they also say it's possible that some of the women took vitamins to remedy health problems or diseases that may have influenced their death risk.