BACKGROUND: The effects of persistence with hormone replacement therapy (HRT) on the risk of hospitalization for cancer and of the route of HRT administration on the risk of breast and colorectal cancer were explored in a large cohort study. PATIENTS AND METHODS: The 73 505 women residing in Lombardia (Italy), aged 45-75 years, who received at least one HRT prescription during 1998-2000 were followed until 2005. Among these, 3687 experienced cancer hospitalization. Proportional hazards model was fitted to estimate the association between cumulative HRT persistence and cancer risk. RESULTS: Compared with women who took HRT for 2 years showed hazard ratios (HR) of 0.78 (95% confidence interval 0.68-0.92) for colorectal cancer and 1.34 (1.13-1.58) for breast cancer. HR for breast cancer associated with long-term use of transdermal and oral HRT were, respectively, 1.27 (1.07-1.51) and 2.14 (1.43-3.21). CONCLUSIONS: Evidence that long-term use of HRT is associated with increased risk of breast cancer and decreased risk of colorectal cancer is supplied from this study from a southern European population. Our findings indicate that transdermal therapy might have lower effect than oral therapy in increasing breast cancer risk.
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