Use of oral contraceptives is not associated with an overall increase in cancer, and it may provide a net public health benefit, reducing women's risk of developing the disease, according to a large British cohort study.
Researchers said the study of more than 1 million woman-years in the Royal College of General Practitioners oral contraceptive study indicates that the overall absolute reduction in risk of any cancer among those who have ever used oral contraceptives (OC) was 45 per 100,000 woman-years in the main data set. The benefit was greater among older women than younger, peaking at age 50–59 (90 per 100,000 woman-years) (BMJ 2007 Sept. 12 [Epub doi:10.1136/bmj.39289.649410.55]).
Researchers, led by Dr. Phillip C. Hannaford of the University of Aberdeen (Scotland), found the risk of all cancers was significantly lower among women who have used oral contraceptives, compared with those who never took them (adjusted relative risk 0.88). Statistically significant risk reductions were detected for OC users in cancers of the large bowel or rectum (adjusted relative risk 0.72), uterine body (0.58), ovaries (0.54), site unknown (0.64), and other (0.88).
Researchers detected a small but statistically insignificant increase in cancers of the lung, cervix, central nervous system, or pituitary for women who had ever taken oral contraceptives. There was no difference in breast cancer between the ever takers and never takers, although for main gynecologic cancers there was a reduced risk among the ever takers (adjusted relative risk 0.71). Median time of use was 44 months.
When compared with women who had never used OCs, however, women who had used them for more than 8 years had significantly increased risk: for all cancers (adjusted relative risk 1.22), cervix (2.73), and central nervous system or pituitary (5.51). Those same women, however, had a significantly reduced risk of ovarian cancer (0.38). “It is important to remember…,” the researchers emphasized, “that comparatively few women in our study used oral contraceptives for such durations, with less than a quarter of users being at this increased risk.”
While the findings should be good news for older women similar in age to those in this study, the researchers cautioned that the trends they found might not continue.
“Many women, especially those who used the first generation of oral contraceptives many years ago, are likely to be reassured by our results,” wrote the researchers. “Our findings might not, however, reflect the experience of women using oral contraceptives today, if currently available preparations have a different risk to earlier products, or if differences in patterns of usage (such as age at starting oral contraceptives or duration of use) materially affect cancer risk.”
They add: “These results suggest that, at least in this relatively healthy U.K. cohort, the cancer benefits associated with oral contraception outweigh the risks.”
The study enrolled 23,000 women taking oral contraceptives and 23,000 who had never used them over 14 months beginning in May 1968. The mean age at the time of recruitment was 29 years. All the women were married or in a stable relationship. Most were white.
The women were followed until they left the area of their recruiting doctor, the doctor left the study, they began receiving oral contraceptives from another source, they died, or they reached the end of the follow-up in 1996. A total of 26% of the women made it to the end of the follow-up in 1996.
Three-quarters of the original cohort, those whose doctors were still in the study, were flagged in the mid-1970s for National Health Service central registries in England and Scotland so subsequent cancers and deaths could be reported to the study, through December 2004.
The study also included a smaller set of data gathered while the subjects were under direct observation of their general practitioners until they were lost to follow-up, they developed their first relevant cancer, or the observation period ended in 1996.
In that smaller data set of more than 500,000 woman-years, researchers were unable to detect many of the statistical differences found in the larger cohort. However, they detected significantly reduced risk for oral contraceptive users of cancers of the uterine body (adjusted relative risk 0.47) and ovary (0.51).