Collaborative Group on Hormonal Factors in Breast Cancer2021-12-142022-03-182021-12-142022-03-1819960140-6736https://repositorio.sibdi.ucr.ac.cr/handle/123456789/17444The use of female sex hormones as contraceptives began in 1960, since when an estimated 200 million women throughout the world have used them.1 The most widely used type of hormonal contraceptive has been the combined oral contraceptive, which contains an oestrogen and progestagen and is prepared from various compounds in various doses and combinations. Other hormonal contraceptives contain progestagen only, given orally or by injection. Many epidemiological studies have investigated whether hormonal contraceptives might affect breast cancer risk,2 K and the Collaborative Group on Hormonal Factors in Breast Cancer was set up in 1992 to bring together, reanalyse, and publish the worldwide data. The main results are summarised here. Additional results, together with full descriptions of the methods, the studies and the women included, are being published elsewhere.engCANCERENFERMEDADESANTICONCEPTIVOSArticle