Purpose: Prior studies suggest that recent oral contraceptive (OC) use is associated with a modest increased risk of breast cancer among young women. However, the majority of these reports have relied on self-reported use and have not characterized risks associated with newer OC formulations.

Methods: We conducted a nested case-control study among health plan enrollees at a large health maintenance organization, Group Health Cooperative, which serves the greater Seattle-Puget Sound region. Cases consisted of 1,102 women 20–49 years of age diagnosed with invasive breast cancer from 1990–2009. We randomly selected 21,952 controls matched on age, year, and enrollment length. Detailed information on recent OC use, including formulation, dose, and duration was ascertained from electronic pharmacy records. Multivariate-adjusted conditional logistic regression was used to calculate odds ratios (ORs) as estimates of relative risks and 95% confidence intervals (CIs).

Results: Recent OC use (within 1 year of diagnosis) was associated with a 60% (95% CI = 1.3–1.9) increased breast cancer risk. The association was slightly stronger for estrogen receptor (ER) positive compared to ER-negative disease (ER-positive OR = 1.7, 95% CI = 1.3–2.1 and ER-negative OR = 1.2, 95% CI = 0.8–1.8), though this difference was not statistically significant. The ORs varied somewhat by OC formulation, with recent use of OCs containing the progestin ethynodiol diacetate (OR = 2.6, 95% CI = 1.4–4.7) or high dose estrogen (OR = 2.7, 95% CI = 1.2–6.4) associated with particularly elevated risk estimates compared to non-use of OCs in the prior year. In contrast, risk estimates for recent use of OCs with the progestin norgestimate or low dose estrogen suggested either a modest association or no association (OR = 1.2, 95% CI = 0.6–2.2 and OR = 1.0, 95% CI = 0.6–1.7, respectively).

Conclusions: These results suggest that recent use of contemporary OC formulations is associated with an elevated risk of breast cancer among women ages 20–49, with associations varying somewhat by OC formulation. Although breast cancer is rare among young women, the potential risk of breast cancer associated with certain formulations could impact OC recommendations by providers if these findings are confirmed.

This abstract is one of the 20 highest scoring abstracts of those submitted for presentation at the 36th Annual ASPO meeting held March 4–6, 2012, in Washington, DC.