Because many of the risk factors for breast cancer are related to natural hormones, and because OCs work by manipulating these hormones, there has been some concern about the possible effects of medicines such as OCs on breast cancer risk, especially if women take them for many years. OCs were introduced in the 1960s, and sufficient time has now elapsed to allow investigators to study large numbers of women who took birth control pills for many years beginning at a young age and to follow them as they age.
Over the last 15 years, various studies examining the use of OCs as a risk factor for breast cancer have produced inconsistent results. While most have found no overall increased risk for breast cancer associated with OC use, in June 1995 investigators at the National Cancer Institute (NCI) reported an increased risk of developing breast cancer among women under age 35 who had used birth control pills for at least 6 months, compared with those who had never used them. They also saw a slightly lower, but still elevated risk among women ages 35 to 44. In addition, their research showed a higher risk among long-term OC users, especially those who had started to take the pill before age 18.
A later comprehensive analytical report, which included the 1995 study, found that although there was a slightly elevated risk of developing breast cancer in women who were current or recent users of birth control pills, the risk of developing breast cancer returned to a normal level 10 years or more after discontinuation of OCs. These study results are considered to be consistent with those of the 1995 study.
The comprehensive report analyzed the results of 54 studies conducted in 25 countries that involved 53,297 women with breast cancer and 100,239 women without breast cancer. Over 200 researchers participated in this combined exhaustive analysis of their original studies, which represent about 90 percent of the epidemiological studies throughout the world that have investigated the possible relationship between OCs and breast cancer.
The reduction in risk after 10 years or more of being off OCs was consistent regardless of family history of breast cancer, reproductive history, geographic area of residence, ethnic background, differences in study designs, dose and type of hormone, and duration of use. This risk reduction also generally held true for age at first use; however, for reasons that are not yet understood, there was a continued elevated risk among women who had started to use OCs before age 20.
One encouraging aspect of the study is that the slightly elevated risk seen in both current OC users and those who had stopped use within 10 years may not be due to the contraceptive itself. The slightly elevated risk may result from estrogen's potential to promote the growth of breast cancer cells already present, rather than its potential to initiate new malignant changes.
Furthermore, the observation that the slightly elevated risk of
developing breast cancer that was seen in this study peaked during
use, declined gradually after OC use had stopped, then returned to
normal risk levels 10 years or more after stopping, is not consistent
with the usual process of carcinogenesis. It is more typical for
cancer risk to peak decades after exposure, not immediately
afterward. Cancer usually is more likely to occur with increased
duration and/or degree of exposure to a carcinogen. In this
analytical study, neither hormonal concentration nor duration of use
affected the outcomes.
Oral Contraceptives and
Cancer Risk
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Reprinted from the National Cancer Institute, National Institutes of Health
