A drug prescribed to combat brittle bones has been shown to prevent invasive breast cancer.

Scientists stumbled on the discovery while investigating whether the drug, raloxifene, could also protect against heart disease.

Although there was no impact on heart disease, the trial showed that the drug reduced by 55 per cent the risk of invasive breast cancers, which spread through tissue barriers into surrounding areas.

Sold under the brand name Evista, raloxifene is used both to prevent and treat osteoporosis in postmenopausal women. It works by activating “receptors” or molecular switches in bone tissue that normally respond to the female sex hormone oestrogen.

By blocking the receptors, raloxifene may prevent some of the effects of oestrogen that spur cancer growth, scientists say. They believe that raloxifene acts in the same way as the highly successful breast cancer drug tamoxifen.

During the trial, only hormone-sensitive cancers that are fuelled by oestrogen were affected, but they make up the majority of breast cancers. More than 10,000 women with heart disease, or at risk of the condition, took part. Over a period of more than five years, those who took raloxifene were 55 per cent less likely to develop invasive oestrogen-positive breast cancer than those who took a dummy placebo drug.

Raloxifene had no effect on noninvasive cancer or cancers not affected by oestrogen. The findings are reported in the Journal of the National Cancer Institute.

There was a downside to the treatment: women given raloxifene were more likely to suffer blood clots and fatal strokes than those taking the placebo.

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