A large study of tibolone, a drug used to treat menopausal symptoms and to prevent osteoporosis, was halted early after researchers found that the synthetic steroid significantly increased the risk of recurrent breast cancer among survivors of the disease. The study was published in the British medical journal The Lancet Oncology.

Tibolone is licensed for use in 90 countries for alleviating these symptoms, and 55 countries have approved its use in treating osteoporosis, according to the study.

However the drug was shown to increase the chances of breast cancer returning by 40%. Moreover 70% of recurrences involved aggressive, spreading tumours which are invariably fatal.

The tibolone hormone can mimic the effects of both estrogen and progestogen, which means women only need to take the one hormone instead of two.

A total of 3,098 women were assessed of which 1,556 were randomised to tibolone and 1,542 to a placebo. Mean age at entry was 52.7 years, and mean time since surgery was 2.7 years. Of the women receiving tibolone, 237 (15.2%) had a recurrence of their cancer, compared with 165 (10.7%) of the women receiving placebo -- a 40% increased risk for the tibolone recipients.

The increased risk was so pronounced that the trial was stopped six months early. "Although the trial was intended to show the non-inferiority of tibolone compared with placebo, the findings clearly show that -- although effective against hot flushes -- tibolone does increase the risk of breast-cancer recurrence," Peter Kenemans of the VU University Medical Centre in Amsterdam and colleagues said.