Abstract The need to seek improved combined oral contraceptive (COC) efficacy, with fewer health risks and better acceptability, has been ongoing since the introduction of COCs more than 50 years ago. New progestin formulations combined with lower doses of ethinyl estradiol (EE), the predominant estrogenic component of COCs, have reduced the incidence of venous thromboembolism and other negative outcomes of COC treatment. Previous attempts to use endogenous 17β-estradiol (E2 ) instead of EE were limited primarily by poor cycle control. The recent introduction of E2 -based formulations has renewed interest to determine if there are potential benefits of using E2 in COCs. These formulations have been shown to have similar efficacy and cycle control as EE-based COCs. This review provides a brief summary of the pharmacology of EE and E2 , including metabolism, pharmacokinetics and pharmacodynamics, as well as adverse effects of these estrogens.