PGR and breast cancer: Oestrogen or progesterone receptor positive breast cancer (ER/PR+) accounts for 83% of all breast cancer, and endocrine therapies are recommended to minimise risk of recurrence.1 2 Since 2006, aromatase inhibitors (AIs) have been recommended over tamoxifen for postmenopausal women due to greater efficacy.3 However, it has recently been suggested that AI users have a higher risk of subsequent cardiovascular disease (CVD) compared with tamoxifen users.4 It is in unclear whether this reflects cardiotoxicities of AIs or a protective effect of tamoxifen.