Lancet Oncology reports that the risks are so great that a five-year trial by a team of Dutch researchers has had to be abandoned.
The synthetic steroid drug tibolone had previously been widely regarded as being safer than conventional hormone replacement therapy (HRT) among women at risk of breast cancer.
But it has now been shown to increase the chances of the cancer returning by 40%, of which 70% of the cases are aggressive tumours that are invariably fatal.
Tibolone is effective against menopause symptoms such as hot flushes, night sweats and bone loss, has been widely prescribed for women undergoing chemotherapy for the cancer.
The trial had assessed the effect of tibolone in more than 3,000 women who had undergone breast cancer surgery. A total of 237 (15.2%) of the women taking tibolone had a recurrence of cancer, compared with 165 (10.7%) of those prescribed a placebo.
More than 70% of the women were also undergoing adjuvant chemotherapy, but it is not clear how this might have affected the outcomes.
Study leader Professor Peter Kenemens, from the VU University Medical Centre in Amsterdam, said: 'There are insufficient data to establish the safety of tibolone in women who have had breast cancer and do not require or have finished adjuvant therapy.'
Lancet Oncology, 2009 10:2:135-146
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