The effects of exogenous hormones on diseases that are hormonally related, particularly breast cancer, cause concerns. Breast cancer is common and the relation with hormones complex. Millions of women have taken menopausal oestrogen and progesterone (hormone therapy, HT) for prolonged periods to alleviate menopausal symptoms, to prevent osteoporosis, and to prevent coronary heart disease too.

The breast cancer risk of HT has been investigated since the 1970s in some 50 studies.¹ It has been known that it was associated with an increase in breast cancer, but there has been confusion because of the possible differential effect of different hormones as well as of different durations, methods of application, and the effect of past use. It was none the less argued that the risk was small.² It is surely a public health responsibility to clarify these risks as far as possible. This is particularly so when the marketing imperatives are so strong.

Use of combined therapy became common in the 1980s, after years of use of oestrogen alone were found to increase endometrial cancer risk, which the addition of progesterone eliminated. Thus epidemiological data on combined therapy were less mature and often confused with data on unopposed therapy, which had been the subject of most studies. The addition of progesterone could, in theory, have had a differential effect on breast cancer risk in any direction.³

The million women study, published in 2003, …