Menopausal hormone therapy use and breast cancer risk in Australia: Findings from the New South Wales Cancer,Lifestyle and Evaluation of Risk study

Randomised controlled trials and large‐scale observational studies have found that current use of menopausal hormone therapy (MHT) is associated with an increased risk of breast cancer; this risk is higher for oestrogen–progestagen combination therapy than for oestrogen‐only therapy. Our study was designed to estimate MHT‐associated breast cancer risk in a population of Australian women. Data were analysed for postmenopausal women with self‐reported incident invasive breast cancer (n = 1,236) and cancer‐free controls (n = 862), recruited between 2006 and 2014 into a large case–control study for all cancer types, the NSW CLEAR study. Information on past and current MHT use was collected from all participants, along with other lifestyle and demographic factors, using a self‐administered questionnaire. Unmatched multivariable logistic regression was performed, adjusting for socio‐demographic, reproductive and health behaviour variables, body mass index and breast screening history. Compared to never users of MHT, the adjusted odds ratio (aOR) for breast cancer in current users of any type of MHT was 2.09 (95% CI: 1.57–2.78; p < 0.0001) and for past users of any type of MHT was 1.03 (0.82–1.28; p = 0.8243). For current users of oestrogen‐only and oestrogen–progestagen therapy, aORs were 1.80 (1.21–2.68; p = 0.0039) and 2.62 (1.56–4.38; p = 0.0003), respectively. These findings are consistent with those from other international observational studies, that current, but not past, use of MHT is associated with a substantially increased risk of breast cancer.