Objective: [corrected] To investigate the relationship between hormone replacement therapy (HRT), smoking, and cancer incidence.
Methods: Baseline interviews were conducted from 1990 to 1992 in a population-based cohort of 29,508 Swedish women aged 25-65 years with no history of cancer. Cancer incidence in the cohort was assessed through December 31, 1999, with the Swedish Cancer Registry, the Population Census Registry, and the Cause of Death Registry.
Results: When follow-up ended, the cohort included 226,611 person-years. A total of 1145 malignancies were diagnosed (observed), and 1166.6 were expected (standardized incidence ratio 0.98; 95% confidence interval [CI] 0.93, 1.04). Women who had experienced a natural menopause and had ever used HRT had no increased incidence of cancer overall (standardized incidence ratio 1.03; 95% CI 0.88, 1.19). Long-term HRT users who smoked had a decreased incidence of smoking-related cancers, such as the oral cavity, pharynx, hypopharynx, larynx, esophagus, lung, cervix, and bladder (standardized incidence ratio 0.24; 95% CI 0.08, 0.76). The effect was seen regardless of the type of HRT (progestin versus non-progestin-containing preparations) used and number of cigarettes smoked. The protective role of HRT for colon cancer was evident among both smokers and nonsmokers. An increased incidence of endometrial cancer was seen only for nonsmokers who used HRT.
Conclusion: Our data indicate that HRT use protects women against smoking-associated cancers. This effect occurs regardless of the type of HRT and the amount of smoking.