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  • Title: Oral contraceptive agents and breast cancer: a population-based case-control study.
    Author: Rohan TE, McMichael AJ.
    Journal: Med J Aust; 1988 Nov 21; 149(10):520-6. PubMed ID: 3185321.
    Abstract:
    In this population-based case-control study that was conducted in Adelaide, South Australia, and which involved 395 case subjects and 386 control subjects who were aged 20 years to 69 years, the adjusted relative risk of breast cancer for women who had ever used oral contraceptive agents was 1.06 (95% confidence interval [CI], 0.70-1.60). Relative risks that were associated with use of oral contraceptive agents for one month to 18 months and for 19 months or more before a first pregnancy were 1.09 (95% CI, 0.45-2.62) and 1.67 (95% CI, 0.63-4.42), respectively, but the trend was not statistically significant. Relatively-little variation in risk was found in association with the total duration of the use of oral contraceptive agents and with years since the first and the last use of oral contraceptive agents. When the risk of breast cancer in association with the use of oral contraceptive agents was examined across levels of risk factors of breast cancer (history of benign breast disease, family history of breast cancer and parity), the only relative risk which deviated markedly from unity was that which was associated with use of oral contraceptive agents in women with a history of benign breast disease; however, the relative risk of 1.77 (95% CI, 0.35-8.97) was not statistically significant. In conclusion, the results of this study support those of the majority of previous studies in showing no overall relationship between the use of oral contraceptive agents and the risk of breast cancer. The findings of a population-based case-control study conducted in Australia in 1982-84 suggest that, for women under 70 years of age, the use of oral contraceptives (OCs) is not associated with an increased risk of breast cancer. The analysis involved 395 cases and 386 controls 20-69 years of age. An identical percentage (49%) of cases and controls reported ever use of OCs. The adjusted relative risk of breast cancer for women who had ever used OCs was 1.06 (95% confidence interval, 0.70-1.60). There was no evidence of a dose-response relationship between the total duration of OC use and the risk of breast cancer; the risk of breast cancer did not increase with an increasing interval since 1st use of OCs; and no clear evidence was found of a reduction in risk with an increasing interval since discontinuation of OC agents. Women who 1st had used OCs 19 years ago or more and who had used them for more than 7 years had a lower breast cancer risk than non users (0.33, 95% confidence interval, 0.15-0.70). Also, women who had used OCs within the past 11 years and had used them for more than 7 years had a relative risk of 0.77 (95% confidence interval, 0.42-1.43), while women who had last used OCs more than 11 years ago and for more than 7 years had a relative risk of 0.38 (95% confidence interval, 0.09-1.61). Finally, there was no significant evidence of interactions between breast cancer risk in OC users and various risk factors for breast cancer, namely, parity, a history of benign breast disease, and a family history of breast cancer. Although these findings are in accord with those of most previous investigations, continued monitoring of the effects of OCs on breast cancer risk is mandatory for several reasons: the effects of agents that act at an early stage in the carcinogenic process may not become evident for 20 years or more; progressive reductions have occurred in the ages at which women begin and end OC use; and OC preparations currently available differ markedly from their predecessors.
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