| Summary: | Ritgerðin er lokuð fram í júlí 2011 vegna birtingar greinar í tímariti The use of oral contraceptives (OCs) and hormone replacement therapy (HRT) is common in Iceland. We investigated the influence of these hormonal exposures on breast cancer risk, with emphasis on interaction. METHODS: This is a population-based cohort study on 31,430 Icelandic women aged 40 years or older when visiting the Cancer Detection Clinic in 1979-2008 and answering question on OC and HRT use. Through record linkage of these data to the Icelandic Cancer Registry, we identified women diagnosed with breast cancer during the course of the study. Using Cox regression analyses, we found hazard ratios (HRs) for different aspects of hormone use. RESULTS: 1,182 women in the cohort developed breast cancer during the study period. Compared to those who never used sex hormones, the increase in breast cancer risk was highest for those who had used both OCs and HRT (HR=1.84; 95% CI 1.51-2.26). The HRs were higher for users of combined regimens than for users of estrogen unopposed HRT (HR=2.19; 95% CI 1.76-2.73 vs. HR=1.25; 95% CI 1.03-1.51, respectively). Higher risk was generally associated with longer duration (HR=1.73; 95% CI 1.41-2.13) and with current rather than past HRT use (HR=1.47; 95% CI 1.17-1.84 vs. HR=1.02; 95% CI 0.66-1.58). Former OC users were at greater risk for breast cancer than non-users of OCs, although we found no interaction between OC and HRT use in a Wald test for interaction (p=0,659; 95% CI 0.82-1.36). CONCLUSIONS: We did not observe a statistically significant interaction between OC and HRT use, although past OC use tended to increase the risk among HRT users.
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