Oral contraceptives and breast cancer: a national study.

In a population based case-control study 433 New Zealand women aged 25-54 with newly diagnosed breast cancer were compared with 897 women selected at random from the electoral rolls. The relative risk of breast cancer in women who had ever used oral contraceptives was 0.94 (95% confidence interval 0.70 to 1.25). The relative risk in women aged 25-34 at diagnosis was estimated to be 2.2 (95% confidence interval 0.47 to 9.9) and in older women less than 1. Analyses of risk by duration of use of oral contraceptives, age at first use, and time since first use showed no adverse effect of the pill. In particular, there was no increased risk in women who had used oral contraceptives before the age of 25 or before their first pregnancy, even for prolonged periods. Given the high prevalence of use in New Zealand, this study provides strong evidence against the hypothesis that use of oral contraceptives at young ages increases the risk of breast cancer.