One in three women in Britain aged 50-64 is now taking HRT to ease the symptoms of the menopause and its popularity has doubled since 1990. But doctors are still unsure what to tell women about the risks.
The treatment, once described as "the last frontier to the emancipation of women", is believed to cut the rate of heart disease but increase the risk of breast cancer. It is also regarded as a valuable defence against the bone-thinning disease osteoporosis but may raise the risk of blood clots.
Fears that it also increased the risk of endometrial cancer (of the lining of the womb) have receded since the introduction of combined preparations including progestogen.
The "Million Women" study will be run by the Imperial Cancer Research Fund with the National Health Service breast screening programme. Questionnaires will be sent to all women invited for screening, seeking details of their medical history and the HRT preparation they are taking. The women are aged 50 to 64.
Professor Valerie Beral, head of the ICRF's epidemiology unit at Oxford and principal co-ordinator of the study, said it was fears that HRT might increase the risk of breast cancer, coupled with the huge number of women now taking it - up to 2 million in Britain - that made the investigation essential.
"Britain is the only country in the world that can carry out this study because it is the only one with a large population and a comprehensive national breast screening programme."
Professor Beral said the scale of the study, costing pounds 1m, was necessary to give clear answers. The aim was to recruit 250,000 women by the end of the year and to meet the target of 1 million by 1999. About 6,000 of them are expected to have developed breast cancer by 1999 and a further 6,000 to have died of heart disease by 2002.
The first results are expected in five years. Professor Beral said: "We know there is nothing drastic going on with HRT. We hope this study will provide the facts so that when women are trying to figure out ... whether or not to take it they will have the information they need to choose."
Julietta Patnick, national co-ordinator of the NHS breast screening programme, said the effects of HRT were a major anxiety for women. "We get a million questions a year about it and unfortunately we don't know the answers. We never like to tell women anything without the evidence to back it up."
She said that if HRT increased the risk of breast cancer it could be because it delayed the menopause and the fall in hormone levels associated with it which reduces the incidence of cancer, or it could be because it delays changes in the breast tissue at menopause which reduce the effectiveness of screening.
The pre-menopausal breast has more glandular tissue which is denser, making it harder to spot cancers in screening. The study would aim to answer these questions and whether the type of HRT made any difference.
Gilly Andrews, 49, a nurse who counsels patients on the menopause, has been taking HRT for 10 years since having a hysterectomy. "It has been wonderful. I have no qualms or anxiety about it at all. I see myself slipping into middle age without ever noticing the menopause has come or gone."
She said she advised all her friends to take it and most had followed her example. "I don't know anyone in my personal circle who has tried it and given it up. For the vast majority of women it is the right choice."Reuse content