The effect of patterns of oral contraceptive use on breast cancer risk in young women. The UK National Case-Control Study Group.

The effect of the duration and pattern of oral contraceptive use on breast cancer risk in young women (aged under 36 at diagnosis) has been investigated. Oral contraceptive users were divided into three groups: group 1, continuous users; group 2, interrupted only by pregnancy users; and group 3, intermittent users. There was a clear trend with duration of oral contraceptive use in all three groups of users (P < 0.001 for each category of use) and the relative risks per year of use were similar (1.07, 1.07 and 1.05 in continuous, interrupted and intermittent users respectively). The relative risks for intermittent users and for women who had used oral contraceptives except when pregnant were very similar, but the relative risk for users for more than 8 years was highest for continuous users. The results suggest that the relationship between oral contraceptive use and breast cancer risk is dependent upon the total duration of use and is not modified by the pattern of use.