Background: Use of oral contraceptives and the risk of cancer continues to be an issue.
Material and methods: The Norwegian Women and Cancer Cohort Study was initiated in order to study the use of oral contraceptives and risk of cancer. A total of 102 443 women aged 31-70 were included from 1991 to 1997. In the follow-up analysis through 1999, 851 of the 96 355 eligible women had developed breast cancer, 171 ovarian cancer, 118 cervical cancer, 110 endometrial cancer and 219 colorectal cancer.
Results: The risk of breast cancer increased with the duration of oral contraceptive use (p for trend p = 0.002), while risk of ovarian and endometrial cancer decreased (p for trend p = 0.03 for endometrial cancer and p < 0.0001 for ovarian cancer). No significant association between the use of oral contraceptives and the combined risk of breast/endometrial/ovarian cancer was revealed.
Interpretation: Use of oral contraceptives does not seem to change the overall risk of cancer. The increased risk of breast cancer is compensated by the protective effect in relation to endometrial and ovarian cancer.