Association of Long-term Use of Calcium Channel Blockers (CCB) and Risk of Breast Cancer: A Retrospective Longitudinal Observational Study

Aims: To examine the association between long-term calcium channel blocker (CCB) use and the development of breast cancer.

Data sources: the Australian Longitudinal Study on Women's Health , 45 and Up Study, Rotterdam study and linked administrative data.

Study cohort: Women with self-reported hypertension (HTN) without prior breast cancer.

Harmonisation: Relevant variables will be checked for harmonisation in which variables available in all three cohort will be used in the harmonised analysis. The variables will be checked on the definition, measurement and harmonisation rules. Variables that cannot be harmonised across the cohorts will be used in the cohort specific analyses.

Exposure measurement: The primary exposure is the use of CCB, which will be identified by the anatomical therapeutic chemical code (C08) in the medicine data. Exposure to CCB as well as other antihypertensive (AHT) medicines will be captured separately as the cumulative dose-duration of exposure during follow up. Participants will be stratified in four groups: women with HTN with no AHT use, women with HTN exposed to CCB only, women with HTN exposed to non-CCB and women with HTN exposed to both CCB and non-CCB.