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.