A warning that long-term use of antibiotics may increase the risk of breast cancer is raised by US researchers today.

Breast cancer is the commonest cancer in women worldwide with more than one million cases diagnosed each year. Antibiotics are among the most commonly prescribed medical drugs and are overused in many countries so if the link were proved it would have far reaching public health implications.

A study of more than 10,000 women of whom more than 2,200 had breast cancer found those who had used antibiotics were 50 per cent more likely to have developed the disease.

Use of the drugs by women in the study, all of whom were enrolled in a health maintenance plan in Washington state on America's west coast, varied from one to 500 days and the risk increased with length of use and number of prescriptions.

However, researchers from the University of Seattle, who carried out the study, say it is not clear whether breast cancer was linked with the antibiotics or with the condition which they were being used to treat.

Writing in the Journal of the American Medical Association, Christine Velicer and colleagues say: "The results of this study support the continued need for prudent long-term use of antibiotics and the need for further studies of the association between antibiotic use and cancer risk."

One possibility is that infections for which antibiotics are prescribed might trigger inflam- mation in the breast, leading to cancer. Or biological mechanisms of the antibiotics may have an effect on the breast. If antibiotics were shown to raise the risk of breast cancer that would raise the question of whether they also influenced the occurrence of the disease at other sites in the body, the study shows.

Commenting on the findings, Michelle Barclay of the UK charity Breakthrough Breast Cancer said: "This study provides food for thought. However, caution should be exercised when interpreting results, as the reason for taking antibiotics may be the actual risk factor, rather than the drug itself. Women should not be deterred from taking antibiotics if their GP thinks it is necessary."