Oral Contraceptives
CATEGORY: IARC known, NTP known
USED IN: Medications
Numerous studies have shown an increased risk of breast cancer in women using oral contraceptives (Althuis, 2003; Dai, 2009; Delort, 2007; Kumle, 2002). The risk is greatest among current and recent users, particularly those who have used them for more than five years and especially those who started using birth control pills earlier in life and took them for longer periods of time (Pasanisi, 2009; Rosenberg, 2009). Several studies have shown that women with BRCA1 or BRCA2 mutations (Haile, 2006; Narod, 2002; Pasanisi, 2009; cf. Figueiredo, 2010), as well as women with family histories of breast or ovarian cancer (Haile, 2006; Narod, 2002; cf. Gaffield, 2009), have an increased susceptibility to the risk-inducing effects of oral contraceptive exposures. The data with BRCA carriers support the hypothesis that increases in the penetrance (proportion of women carrying the mutation in which the deleterious effects are expressed) of the mutation are related to exposures to environmental toxicants (King, 2003), especially those that mimic or interfere with natural estrogens.
As with HRT, current use of oral contraceptives has been associated with an increase in breast tumors originating in the lobular tissue (Newcomer, 2003), as well as with the ER– (no or low estrogen-receptor) profile of the disease (Althuis, 2003). Use of oral contraceptives for 10 years or longer has also recently been associated with a diagnosis of comedo DCIS (Phillips, 2009), the most aggressive form of DCIS, which is sometimes confused with early forms of invasive breast cancer (Pervez, 2007).
A recent study examined possible effects of oral contraceptive use on later risk for breast cancer in Hispanic and non-Hispanic white women. Statistically, Hispanic women have somewhat lower rates of breast cancer than do white women, and they are more likely to have breast cancer that is ER+. Despite these group differences, use of oral contraceptives in the past five years is associated with significant increases in breast cancer incidence in both groups. The effect was magnified for women of both groups when oral contraceptive use continued for more than 20 years. Mirroring other study evidence, and again for both Hispanic and non-Hispanic white women, significant increases in ER– tumors were observed following prolonged oral contraceptive use (Sweeney, 2007).
Post-menopausal women who used oral contraceptives for eight or more years but have discontinued use for at least a decade show no significant increase in breast cancer rates (CGHFBC, 1996; Vessey, 2006).
