"Even if we do not know whether there is a causal link, our results are an alarm signal which must be taken seriously and researched further," said Susanna Larsson at Karolinska Institutet to Uppsala Nya Tidning.
35,000 women in Uppsala and Västmanland county took part in the study which has been published in the American Journal of Clinical Nutrition.
None of the women had cancer when the study was launched at the end of the 1990s; during the following ten years 974 contracted the condition.
When researchers looked at those afflicted it was found that the women who consumed multivitamin pills were over-represented. Among the women 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.
"There are studies which show that women who eat multivitamin pills get firmer breasts than other women. And firmer breasts are a known risk factor for breast cancer," said Larsson.