Menopausal hormone usage and breast cancer in Saskatchewan: a record-linkage cohort study.

The association between the occurrence of carcinoma of the breast and previous usage of menopausal estrogens, progestins, and oral contraceptives is examined in a record-linkage study using the Saskatchewan Health Prescription-Drug-Plan Database. Saskatchewan Health is a governmental agency that funds publicly insured health care for essentially all residents of the province. For this study, all women aged 43-49 years in 1976 resident in Saskatchewan were identified from the Saskatchewan Health master registration file. These women were linked by registration beneficiary number to the Drug-Plan Database for the period from January 1976 to June 1987 and to the Provincial Cancer Registry Database for the period from March 1960 to December 1990. The fact and date of death or emigration from the province were obtained through the annual updates of the health plan. Of the 33,003 women initially in the cohort, 213 had a breast cancer diagnosed before 1976 and were omitted from this analysis. Between 1976 and 1990, 742 new primary breast cancer cases occurred. Women taking estrogens unopposed by progestins had an elevated risk of breast cancer, the risk increasing by 7% (relative risk = 1.072, 95% confidence interval 1.02-1.13; p = 0.008) for each 252 tablets used (approximately 1 year of use). Usage of estrogens opposed by progestins showed no association with risk (p = 0.48). Women taking oral contraceptives during this follow-up period also had a higher risk, increasing by 14% (relative risk = 1.144, 95% confidence interval 1.05-1.24; p = 0.002) for every 252 tablets used. These 1-year risk elevations are small but become appreciable at longer durations. For example, at 5 years of unopposed estrogen use, the relative risk is 1.42; for 5 years of oral contraceptive use, it is 1.96.