Oral Contraceptives
What is an exogenous estrogen?
An endogenous estrogen is one that occurs naturally within the body. In contrast, an exogenous estrogen is one that comes from outside the body -- from a drug or from foods with estrogen-like properties. The exogenous estrogens used in birth control pills and estrogen replacement therapy may be manufactured from natural or synthetic sources. In the United States, the most common form of replacement estrogen is a natural estrogen called conjugated equine estrone (e.g., Premarin) -- "equine" because it is derived from the urine of pregnant mares. Some estrogen replacement drugs are made from plant estrogens (e.g., Ogen).
Does the use of oral contraceptives increase my risk of breast cancer?
In theory, birth control pills, like any other source of estrogen, could affect the risk of breast cancer. However, the degree of real risk is not clear. There are a number of confusing factors. First, women are more likely to use birth control pills when they are younger and at a lower risk of breast cancer anyway. Any extra risk associated with the use of oral contraceptives -- if it exists -- disappears entirely within 10 years of discontinuation of use.
Women who are at the highest risk of getting breast cancer (those over the age of 50) are more likely to have stopped taking the pill sometime earlier and would no longer be affected by the associated risks. Second, for a variety of reasons, women who use birth control pills often have a different risk profile than women who do not use the pill. This makes it more difficult to calculate the exact risk of using oral contraceptives. For example, women who take the pill are more likely to have children later in life, a fact that does affect the risk of breast cancer.
Finally, women on birth control are more likely to undergo some kind of screening for the disease and are therefore are more likely to be diagnosed with breast cancer. For example, women on oral contraceptives are more likely to visit their doctors for routine exams, because most doctors require an examination before renewing a prescription for the pills. Therefore, a breast cancer is more likely to be identified and diagnosed in women who take birth control pills.
What is the evidence on the relationship between the risk of breast cancer and the use of oral contraceptives?
Studies show an average increase of 24% in the risk of breast cancer during the years when women are using oral contraceptives (a time when the rates of breast cancer are quite low). This increase in risk has been consistent in 54 studies of 53,000 women who had breast cancer (25 countries were represented).
For every 10,000 women who use the pill to age 30, there are an estimated five more diagnoses of breast cancer made from the beginning of use through age 40 than are estimated to occur in women who never took the pill. The excess risk -- if it exists -- disappears entirely within 10 years of discontinuation of use.
If studies consistently show a higher risk of breast cancer risk for women on birth control pills, why are there doubts about the relationship between the two?
If 54 studies of 53,000 women with breast cancer in 25 countries consistently say that birth control pills increase a woman's risk of breast cancer by 24%, doesn't that mean oral contraceptives cause breast cancer? No. The data are actually very confusing. For starters, women who choose to use birth control pills tend to be different from women who do not, in terms of education, income, lifestyle, and a variety of other factors associated with the risk of breast cancer.
Just as important, women on the pill are more likely to be diagnosed with breast cancer earlier. Women on the pill get regular checkups-if for no other reason than to renew prescriptions. Since most checkups include a breast examination, this increases the chances that an early cancer will be identified.
If slowly growing lumps were identified even nine months earlier in the developmental process among women in their twenties who used the pill, this average difference would appear to be a 25% increase in breast cancer compared to the rate for nonusers. There is some evidence that the studies on the use of birth control pills reflect this kind of effect, which is called a "lead-time bias." For example, breast lumps diagnosed as cancer among women who use the pill are less likely to have spread beyond the breast, which means that these cancers were found at an earlier stage.
Finally, if the pill directly increased the risk of breast cancer, one might expect that women who used higher doses of stronger estrogens for longer periods of time (or who began using estrogens at an earlier age) would have breast cancer more often than short-term users of low-dose estrogens. However, no consistent difference has been found for any of these factors.
