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A National Cancer Institute-funded case control study suggests that some current formulations of oral contraception (OCs) may increase risk of breast cancer.
A National Cancer Institute-funded case control study suggests that some current formulations of oral contraception (OCs) may increase risk of breast cancer. The authors note, however, that their findings require confirmation and should be interpreted with caution, given the established health benefits of OCs.
Published in Cancer Research, the nested case-control study led by researchers at Fred Hutchinson Cancer Research Center was among female enrollees in a large US integrated health care delivery system. Detailed information on OC use such as drug name, dosage, and duration of use was gathered from electronic pharmacy records, unlike previous studies that relied on self-report or recall.
Cases (1,102 women aged 20 to 49 years diagnosed with invasive breast cancer from 1990 to 2009) were matched to randomly sampled controls (n=21,952) from enrollment records based on age, year, enrollment length, and medical chart availability. Conditional logistic regression was used to analyze OC use information from the electronic pharmacy records.
Women who had used OCs within the prior year had an increased risk of breast cancer (OR, 1.5; 95% CI, 1.3-1.9) compared with those who had never used OCs or were former users. The association was stronger for estrogen receptor (ER)-positive disease (OR, 1.7; 95% CI, 1.3-2.1) than for ER-negative disease (OR, 1.2; 95% CI, 0.8-1.8) although it did not reach statistical significance (P=0.15). Particularly elevated risks were associated with recent use of OCs containing high-dose estrogen (OR, 2.7; 95% CI, 1.1-6.2), ethynodiol diacetate (OR, 2.6; 95% CI, 1.4-4.7), or triphasic dosing with an average of 0.75 mg of norethindrone (OR, 3.1; 95% CI, 1.9-5.1). OCs containing low-dose estrogen were not associated with elevated risk (OR, 1.0; 95% CI, 0.6-1.7).
In evaluating these findings, it should be noted that there are decades of negative studies regarding OCs and breast cancer risk and, because the results are from a case control study, they are subject to ascertainment bias. If the findings are confirmed, however, the authors said “consideration of the breast cancer risk associated with different oral contraceptive types could impact discussions weighing recognized health benefits and potential risks.”
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