We investigated the survival of female patients with pre-existing type 2 diabetes (T2D) diagnosed with invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) of breast, in relation to the use of metformin, other antidiabetic medication (ADM) and statins. The study cohort consisted of 3,165 women (2,604 with IDC and 561 with ILC). The cumulative mortality from breast cancer (BC) and from other causes was calculated using the Aalen-Johansen estimator. The cause-specific mortality rates were analysed by Cox models, and adjusted hazard ratios (HRs) were estimated for the use of different medications. No evidence of an association of metformin use with BC mortality was observed in either IDC (HR 0.92, 95% confidence interval [CI] 0.64–1.31) or ILC (HR 0.68, 95% CI 0.32–1.46) patients, when compared to other oral ADMs. The mortality from other causes was found to be lower amongst the IDC patients using metformin (HR 0.64, 95% CI 0.45–0.89), but amongst ILC patients the evidence was inconclusive (HR 1.22, 95% CI 0.64–2.32). Statin use was consistently associated with reduced mortality from BC in IDC patients (HR 0.77, 95% CI 0.62–0.96) and ILC patients (HR 0.59, 95% CI 0.37–0.96), and also mortality from other causes in IDC patients (HR 0.81, 95% CI 0.67–0.96) and in ILC patients (HR 0.66, 95% CI 0.43–1.01). We found no sufficient evidence for the possible effects of metformin and statins on the prognosis of BC being different in the two histological subtypes.
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