To determine whether use of sex steroid hormones for contraception and hormone replacement therapy alters the risk of breast cancer, and whether the risk varies with their composition, duration of use, the period of a woman's life when the hormones are used, and after successful treatment for breast cancer.
The results of important epidemiological reports, readily available from the English literature and published since 1981, were evaluated, using reports of basic scientific work as a background to the problem.
An attempt was made to obtain most of the relevant reports. Twenty case-control and seven cohort studies were available on the oral contraceptive pill (OCP) and eleven case-control and five cohort studies on hormone replacement therapy (HRT).
The relative risk estimates for breast cancer (and their 95% confidence intervals) determined by each report were tabulated according to the specific conditions of analysis, for example users under age 25, duration of use. Results by meta-analysis from previous studies were also used to determine risk. A significant positive association was present when the risk estimate exceeded 1.0 and the 95% confidence interval did not cross 1.0.
Among OCP users, the vast majority of reports showed no significant risk of breast cancer--overall, longest duration of use, and use before first full-term pregnancy. However, a positive association between breast cancer and users under age 25 was found in three of eight reports. Similarly, the majority of reports showed no significant risk of breast cancer among HRT users, overall as well as in relation to duration of use and interval since first use. There was no increased risk with additional progestogen; it may be protective. An improved prognosis was found in users who developed breast cancer. On the limited data, use of hormones for postmenopausal symptoms did not appear to be harmful to women who had been successfully treated for breast cancer.
The review revealed good evidence that use of sex steroid hormones had no significant effect on the risk of breast cancer, whether given for contraception or hormone replacement. There was some concern about increased risk with prolonged use of the OCP, especially in younger women. At present, use of these hormones is a matter of informed choice, with individual considerations of the risk-benefit ratio.