It's well known that the primary result of taking vitamins is basically expensive urine (e.g. here), despite alternative medicine sites claiming it's a myth.
However, now there may potentially be a much more serious downside to multivitamins than just expensive urine - breast cancer.
A study published in the American Journal of Clinical Nutrition studied 35,000 women over a 10-year period.
In 1997, 35,329 cancer-free women completed a self-administered questionnaire that provided information on multivitamin use as well as other breast cancer risk factors.
During a follow-up, 974 women were diagnosed with incident breast cancer. When researchers checked, they found that women using multivitamins were over-represented. For those taking at least seven vitamin pills per week, the risk of contracting breast cancer increased by 19 percent in comparison with those who did not take vitamins at all.
The study concluded that the results suggest multivitamin use is associated with an increased risk of breast cancer, and that this observed association is of concern and merits further investigation.
It is known that some women who use multivitamins get firmer breasts than other women, through an increase in breast tissue density, and significant increases the density of breast tissue is a strong risk factor for breast cancer.
This is not a causal link, but, as the study concludes, needs further investigation.
Complimentary health is a A$2.5 billion industry in Australia.
Multivitamin use and breast cancer incidence in a prospective cohort of Swedish women. Susanna C Larsson, Agneta Åkesson, Leif Bergkvist and Alicja Wol. American Journal of Clinical Nutrition, March 24, 2010. doi:10.3945/ajcn.2009.288