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Hannaford et al reported a significant 12% reduction in the risk of any cancer (adjusted relative risk 0.88, 95% confidence interval 0.83 to 0.94), which was widely cited in the popular media as reassuring evidence of the safety of oral contraceptives.1 But although it was prudent to exclude participants under age 38 at time of loss to follow-up, since use of oral contraceptives after that time would be unknown, the authors selectively excluded only non-users at the time of loss to follow-up.
In their discussion, Hannaford et al report that an analysis of the data with all participants under 38 at time of loss to follow-up excluded gave a null result (0.95, 0.88 to 1.02). Hence they disproved their own overall result, clearly showing that their significant overall protective effect of oral contraception was an artefact resulting from the biased exclusion criterion.
Furthermore, they report an increase in breast cancer risk, peaking (relative risk 2.45) between 15 and 20 years after cessation of use instead of disappearing 10 years after cessation of use, as others have reported.2 They also report a significant risk increase (1.22) for any cancer and for breast cancer with more than eight years of using oral contraceptives. Although they note that fewer than a quarter of users in their study had used oral contraceptives for that long, current patterns of use are usually for much longer periods and also more often start before first full term pregnancy, a use pattern producing threefold increases in the risk of breast cancer.3 A further finding is the strong association for cancers of the central nervous system or pituitary, with the relative risk for these cancers steadily rising to 5.51 with more than eight years’ use.
Their conclusion that the cancer benefits associated with oral contraception outweigh the risks is therefore irresponsible, as their results imply the opposite.
Competing interests: None declared.