ABSTRACT
The Collaborative Group assembled and analysed 90% of the worldwide epidemiological data on the relation between risks of breast cancer and the use of hormone replacement therapy (HRT). This represents results on more than 160 000 women world-wide derived from 51 different epidemiological investigations spanning a 25-year period. The main findings are that the risk of breast cancer: (i) is significantly increased in women using HRT (by a factor of 1.023) and (ii) increases with increasing duration of use, but (iii) the excess risk is reduced after discontinuation of use (and has largely disappeared after 5 years); (iv) the relative risk among current/recent users is greater for women with low weight; (v) the breast cancers diagnosed in women who had used HRT were less advanced clinically than those diagnosed in newer users. The authors discuss confounding factors and potential biases within their analyses, the consistency of effect within individual studies, the explanation for the finding and the need to balance the increased risk to breast cancer with other risks/benefits of HRT.
