The finding of an oral contraceptive effect on breast cancer, however small and whether harmful or beneficial, could throw some general light on the pathogenesis of this devastating disease. Although a comprehensive evaluation of the possible etiologic mechanisms of breast cancer is beyond the scope of this report, clearly the uncertainty that currently exists about a possible interaction between this too-common cancer and the method of contraception used at one time or another by 80 percent of women in the United States must be resolved.
Even though, over the past 30 years, oral contraceptives have become one of the most intensively studied drugs in history, the observational data collected to date are inadequate to answer the basic questions of whether oral contraceptives have an effect on the course of breast cancer and whether they make it more common or less common. About a dozen factors are at the core of what is—and is not—known.
The United States has one of the world's highest annual rates of breast cancer incidence, a rate that was already increasing steadily when oral contraceptives were introduced. Whether susceptible women had already fulfilled their carcinogenesis potential (see Appendix B) and could not react further to an added carcinogen, or whether it will simply take many more observations to measure a relatively small increment against an escalating high background rate, is hypothetical.
The age at which women have chosen to start using oral contraceptives has decreased (Table 1-4), and their pattern of use before the first pregnancy (Table 1-5) and to space pregnancies has changed. Younger women will need effective contraception for more than the average 5 years' use by older birth cohorts (Table 1-6). Studies to date have largely focused on patterns of use in years past and, although reassuring overall, do not address more recent patterns of use. In fact, the best existing information about oral contraceptives is for women now in their late 40s and early 50s— the first generation of oral contraceptive users in the 1960s—who used the pill to space their second and later pregnancies. Exposure to oral contraceptives around the time of menarche or menopause, or preceding the first full-term pregnancy, may have quite different influences.
Neither a positive nor negative association between overall breast cancer risk and oral contraceptive use has been found among the first generation of oral contraceptive users from the 1960s. These women lived in developed countries and used the pill for relatively short