Presently, the lowest effective estrogen dose available as a combination oral contraceptive (OC) in the United States is 20 microg of ethinyl estradiol (EE) with different progestins. Twenty micrograms of EE coupled with levonorgestrel results in fewer side effects and cycle control comparable with higher-dose pills. Differences between therapeutically equivalent and brand-name, low-dose oral contraceptives, with respect to the bioavailability of hormones, may interfere with contraceptive efficacy and increase breakthrough bleeding. One of the most common reasons why women discontinue OCs is increased breakthrough bleeding. Because after OC discontinuation most women switch to a less-effective method, or no method, of contraception, an increase in breakthrough bleeding could ultimately result in an increase in unintended pregnancy. Thus, substituting a therapeutically equivalent for a brand-name low-dose oral contraceptive may have significant clinical and economic effects on individual and public health.