When it comes to vitamins, it appears you could have too much of a good thing, say researchers who report a link between their use and higher death rates among older women.
Experts have suspected for some time that supplements may only be beneficial if a person is deficient in a nutrient.
And excess may even harm, as the study in Archives of Internal Medicine finds.
All of the women, in their 50s and 60s, were generally well nourished yet many had decided to take supplements.
Multivitamins, folic acid, vitamin B6, magnesium, zinc, copper and iron in particular appeared to increase mortality risk.
The researchers believe consumers are buying supplements with no evidence that they will provide any benefit.
They are quick to stress that their study relied on the 38,000 US women who took part in it recalling what vitamins and minerals they had taken over the previous two decades.
And it is difficult to control for all other factors, like general physical health, that might have influenced the findings.
But they say their findings suggest that supplements should only be used if there is a strong medically-based cause for doing so because of the potential to cause harm.
“Based on existing evidence, we see little justification for the general and widespread use of dietary supplements,” Dr Jaakko Mursu of the University of Eastern Finland and his research colleagues said.
In the study, iron tablets were strongly linked with a small (2.4%) increased death risk, as were many other supplements. The link with iron was dose-dependent, meaning the more of it the individual took, the higher their risk was.
Conversely, calcium supplements appeared to reduce death risk. However, the researchers say this finding needs more investigation and they do not recommend that people take calcium unless advised to by a doctor in order to treat a deficiency.
Drs Christian Gluud and Goran Bjelakovic, who review research for the Cochrane Database of Systematic Reviews to evaluate best evidence, said: “We think the paradigm ‘The more the better’ is wrong.”
They say dietary supplementation has shifted from preventing deficiency to trying to promote wellness and prevent diseases, and caution: “We believe that for all micronutrients, risks are associated with insufficient and too-large intake.”
Helen Bond of the British Dietetic Association said some people, like the elderly, might need to take certain supplements. For example, vitamin D is recommended for people over the age of 65.
But she said that generally, people should be able to get all the vitamins and minerals they needed from a healthy, balanced diet.
She said some took supplements as an insurance policy, wrongly assuming that they could do no harm. “But too much can be toxic and it is easy to inadvertently take more than the recommended daily amount.”