I kicked the vitamin habit some years ago.
The belief that supplemental vitamins have any benefit for health is not
being backed up by the largest and best-designed studies.
This article sums up my own views nicely: Science-Based Medicine Vitamins and Mortality
A couple of extracts:
Today vitamins have a deserved reputation for being an important part of overall health. However, their reputation has gone beyond the science and taken on almost mythical proportions. Perhaps it is due to aggressive marketing from the supplement industry, perhaps recent generations have grown up being told by their parents thousands of times how important it is to take their vitamins, or eat vitamin-rich food...
...Regardless of the cause, the general feeling is that vitamins are all good – they are not only important for health, they promote health. Many people take vitamin supplements on the idea that more is better, or for nutritional “insurance” to make sure they are getting enough of every vitamin.
The problem with deeply embedded cultural beliefs is that people make decisions based upon assumptions that everyone “knows,” rather than making evidence-based risk vs benefit decisions. This phenomenon is exacerbated when the industry is able to make aggressive health claims without requiring any scientific evidence to back up those claims (as is the case in the US since DSHEA was passed in 1994).
It is therefore important to shatter the pedestal on which vitamins have been placed, to bring them down to the level of scientific evidence. The good news is, there is a ton of research on vitamins, which continue to be the subject of much new research.
The article goes on to summarise a long term study that found:
...a small but statistically significant increase in mortality for those taking multivitamins, B6, folic acid, iron, copper, magnesium and zinc. There was also a small decrease in mortality for those taking calcium.
The article goes on to discuss some of the limitations of the study. However, this is by no means the only large scale study reporting increased mortality associated with supplemental vit use. A Cochrane review of available evidence a few years back concluded that there was not much evidence that supplemental vits do any good and some evidence that they may do harm.