Women who regularly take multi-vitamin pills face a much higher risk of breast cancer, a study has found.

The Swedish study, which looked at more than 35,000 women aged between 49 and 83 during a 10-year period, found that women who take daily multi-vitamin pills are nearly 20 per cent more likely to develop breast cancer.

Researchers at Karolinska Institute in Stockholm said a "biologically plausible" explanation is that supplements may trigger growth by increasing the density of breast tissue, which increases the risk of cancer. Folic acid, often present in multi-vitamins, may also accelerate tumour growth.

Although the result was concerning, researchers said more in-depth investigation was needed as to whether or not multivitamins were safe.

"These results suggest multi-vitamin use is associated with an increased risk of breast cancer. This is of concern and merits further investigation," researchers told the American Journal of Clinical Nutrition.

They said it was worrying because many women used these supplements in the belief that they prevented diseases such as cancer. But the researchers stressed that the risks to women on an individual basis remain small, and a vast majority of multi-vitamin users will not develop cancer.

Breast cancer is the most common cancer among New Zealand women, and every year, about 2500 will be diagnosed with the disease. A woman has a one in nine chance of developing it at some point in her life, and more than 90 per cent diagnosed with the disease will have no family history of breast cancer.

It is the leading cause of cancer-related death for women here, with more than 600 dying from the disease annually.

The Cancer Council said the study added to a growing body of evidence that multi-vitamins were "not all they're cracked up to be", the Sunday Telegraph reported.

Cancer Research UK's science information manager Kat Arney told the Times of India that people could get all the nutrients that need from a healthy balanced diet, rich in fruit and vegetables.

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* 35,000 women studied over 10 years
* Breast cancer is the most common cancer among NZ women
* 1 in 9 women will have breast cancer in their lifetime
* 600 NZ women will die from the disease this year