Women who stay on hormone replacement therapy (HRT) for more than 10 years increase their risks of developing fatal ovarian cancer by 70 per cent, a major new study has discovered.

So overwhelming is the evidence that doctors may be advised to tell patients of the increased risks if the drug is taken long-term. This is a big blow to the therapy because it is viewed as a long-term treatment. Several earlier, and smaller, studies had even thought the therapy protected against ovarian cancer, but none had looked at the long-term effects.

The discovery has been made by the American Cancer Society which monitored 240,000 women who were about to start, or had just started, menopause. Of these, 68,906 were on estrogen replacement therapy or HRT. In the seven years they were tracked, 436 died from ovarian cancer, and 142 of these had used HRT.

The research discovered that the risk increased the longer the women were on HRT, and it became significant among those who had taken the drug for longer than 10 years.

The researchers note, however, that the study was carried out when the normal estrogen replacement therapy dosage was 1.25 mg; this dosage has recently been halved, which researchers presume could reduce the risks, although this also needs to be studied (American Journal of Epidemiology, May 1995).