Oral contraceptive use and breast cancer in young women. A joint national case-control study in Sweden and Norway

Abstract

The possible association between oral contraceptive (OC) use and the risk of breast cancer developing before the age of 45 was investigated by means of a population based case-control study in Sweden and Norway. Information was obtained by personal interview from 422 (89.2%) of all eligible patients with a newly diagnosed breast cancer from May, 1984, to May, 1985, and from 722 (80.6%) of all contacted age-matched controls. A multivariate analysis, which accounted for several possible confounding factors, revealed a significant (p = 0.03) association between total duration of OC use and breast cancer risk. The relative risk (RR) of breast cancer after 12 or more years of OC use was 2.2 (1.2-4.0). OC use for more than 7 years before first full-term pregnancy entailed an increased breast cancer risk (RR = 2.0 [1.0-4.2]) which was of borderline significance. When total duration of use was considered, the risk of breast cancer was virtually unrelated to age at first OC use and latency from first use. The results suggest that long-term use of OCs may increase the risk of breast cancer in young women.

PIP: The possible association between oral contraceptive (OC) use and the risk of breast cancer before the age of 45 was investigated by means of a population based case-control study in Sweden and Norway. Interviews were conducted with 422 (89%) of eligible patients with breast cancer diagnosed from May 1984-May 1985 and with 722 age-matched controls. 81% of cases and 77% of controls in Sweden and 66% of cases and 61% of controls in Norway reported ever-use of OCs. A multivariate analysis, which accounted for several possible confounding factors (age at menarche, age at 1st full-term pregnancy, parity, manopause, history of benign breast disease, and family history of breast cancer), revealed a significant (p = 0.03) association between total duration of OC use and breast cancer risk. The relative risk of breast cancer after 12 or more years of OC use was 2.2 (95% confidence interval, 1.2-4.0). OC use for more than 7 years before 1st fullterm pregnancy entailed a breast cancer risk of 2.0 (1.0-4.2), which was of borderline significance. The total duration of OC use showed a consistently stronger association with breast cancer risk than either recency of OC use or time from 1st use to diagnosis of breast cancer. These results suggest that OC use of 8 years' duration or more is associated with an increased risk of premenopausal breast cancer.