Background: As part of the Oxford Family Planning Association study, we compared mortality in relation to oral contraceptive use and smoking to highlight the differences between them from the perspective of public health.
Methods: The study consisted of 17032 women, aged 25-39 years at entry, recruited between May 1, 1968, and July 31, 1974, who had used oral contraceptives, a diaphragm, or an intrauterine device. We assessed mortality from follow-up data recorded until Dec 31, 2000. The analysis is based on woman-years of observation.
Findings: We analysed 889 deaths. Women who had ever used oral contraceptives had increased mortality from cervical cancer (rate ratio 7.2, 95% CI 1.1-303), and decreased mortality from other uterine (0.2, 0.0-0.8) and ovarian cancers (0.4, 0.2-0.7). Oral contraceptives had some adverse effect on deaths from ischaemic heart disease in women who smoked 15 or more cigarettes per day. For all causes of mortality, the rate ratio for death in women who ever used oral contraceptives was 0.89 (95% CI 0.77-1.02). By contrast, this rate ratio was 1.24 (1.03-1.49) in those who smoked one to 14 cigarettes per day, and 2.14 (1.81-2.53) in those who smoked 15 or more cigarettes per day.
Interpretation: There was no harmful effect of oral contraceptive use on overall mortality. By contrast, death from all causes was more than twice as high in smokers of 15 or more cigarettes a day as in non-smokers. The harmful effect was already apparent in women aged 35-44 years.