A drug tipped for widespread use to prevent breast cancer in post-menopausal women also accelerates loss of bone density, thus potentially boosting the risk of fractures, a new study says.

Exemestane - brand name Aromasin - is part of a drug class called aromatase inhibitors, which lower levels of the oestrogen that some breast cancers need in order to grow.

Overall, the study by Canadian bone specialists showed that the drug was highly effective, reducing the risk of breast cancer by almost two-thirds.

The bone sub-study looked at 351 women who had been taking either exemestane or a dummy lookalike pill and whose bone density was measured with hi-tech scanners.

After two years, women taking exemestane had a high loss of bone density at a common fracture point in the wrist called the distal radius, and also at the lower end of the tibia, compared with their counterparts on placebo.

The exemestane users also had an 8 percent decline in the thickness of cortical bone, which comprises the protective outer shell of the bones.

That finding in particular is worrying because nearly four out of every five fractures in old age occur in cortical bone. These accidents are a huge source of disability.