Selection of cases
The study entry criteria for cases were the record of first oral or transdermal HRT prescription at age between 46 and 65 years. Cases were either oestrogen-only or combined HRT users. The British National Formulary (BNF) drug codes were used to identify the patients on HRT.28 Cases were classified as combined HRT users if they received oestrogen and progesterone either in a one single prescription or in two separate prescriptions.