Women who take multi-vitamin pill daily are nearly 20 per cent more likely to develop breast cancer, revealed a major study.

The shocking finding has rattled Australia's $2.5 billion complementary health industry, which is advising consumers not to panic, says News Ltd.

Based on a 10-year study of more than 35,000 women, scientists have found those who regularly took a multi-vitamin pill had 19 per cent increased risk of developing a tumour.

The result was worrying and further investigations are required as many women take multi-vitamins believing they prevent chronic diseases such as cancer.

A biologically plausible explanation is that taking vitamin and mineral supplements significantly increases the density of breast tissue, a strong risk factor for breast cancer.

One key ingredient, folic acid which is present in multi-vitamins may also accelerate tumour growth.

The research led by Sweden's Karolinska Institute and published in the American Journal of Clinical Nutrition, has been welcomed with interest and caution by experts in Australia.

The study showed that women who took a multi-vitamin pill had higher breast tissue density than those who took no vitamin supplements.

Susanna Larrson, lead author of the study said, Results from this prospective study suggest that the use of multi-vitamins may increase the risk of breast cancer.