Statin use and risk of contralateral breast cancer: an updated cohort study with landmark analysis.

Statins may have anti-cancer effects against breast cancer, but evidence regarding their influence on the risk of contralateral breast cancer (CBC) remains inconclusive. In this updated study, incorporating a larger sample, extended follow-up, and landmark analyses, we reevaluated the association between post-diagnosis statin use and CBC incidence among women with breast cancer. Utilising the Danish Breast Cancer Group clinical database, we ascertained data on a nationwide cohort of women aged ≥ 20 years and diagnosed with primary invasive unilateral breast cancer between 1996 and 2019. Data on tumour characteristics, drug use, primary breast cancer therapy, and socioeconomic parameters were retrieved from nationwide health and administrative registries. Using Cox regression, we estimated multivariable-adjusted hazard ratios (HRs) with 95% confidence intervals (CIs) for CBC through 2021 associated with post-diagnosis statin use (≥ 1 prescription) defined at landmark time points of 1, 5, and 10 years after first primary breast cancer. Follow-up began at each respective landmark. Among 77,675 women with breast cancer, 2758 were diagnosed with CBC during median follow-up of 7.2 years (interquartile range 3.4-12.0 years) from the 1-year landmark. Post-diagnosis statin use was not associated with the rate of CBC at any of the landmarks (1-year: HR, 1.01; 95% CI 0.88-1.16; 5-year: 1.08; 0.95-1.24; 10-year: 0.95; 0.80-1.12). Additionally, we observed no consistent trends with duration or consistency of post-diagnosis statin use. Stratification by oestrogen receptor status and pre-diagnosis statin use had no substantial influence on the associations. In conclusion, our study did not support an inverse association between statin use and CBC incidence following a breast cancer diagnosis.