Women who take hormone replacement therapy are more at risk of ovarian cancer and of dying of the disease, a new study has found.

Results published in The Lancet from the Million Women Study, the largest of its kind undertaken, show that over a five-year period there is one extra case of ovarian cancer in every 2,500 women on HRT. And for every 3,300 women who take the drug there will be one additional death.

Researchers studied a quarter of all women aged between 50 and 64 in the UK over a seven-year period. Of these 950,000 women, about a third were taking HRT and a further fifth had taken it in the past. They concluded that use of HRT since 1991 is likely to have resulted in an extra 1,300 cases of ovarian cancer and in 1,000 deaths. The risk of contracting the cancer was increased regardless of which kind of HRT the women were taking.

Ovarian cancer is the fourth most common cancer in women in the UK, with almost 7,000 new cases each year. The five-year survival rate for the disease is less than 30%.

Other cancer implications

Previous results from the Million Women Study have linked the use of HRT with breast and endometrial cancer. Together with ovarian cancer, these three diseases account for almost 40% of all cancers in women in the UK and 25% of cancer deaths. The overall incidence of these cancers in women who take HRT is 31 cases for every 1,000 women over five years. This compares with 19 cases in women who have never taken HRT.

On the back of these findings, Professor John Toy, UK medical director of Cancer Research, warned: “Women should think very carefully about whether to take HRT. And women who choose to take HRT should aim to do so for clear medical need and for the shortest possible time.”