Disparities in Emergency Contraceptive Metabolism Dictate Efficacy

Inclusion Criteria: * Generally healthy women * Aged 18-40 * regular menses (every 21-35 days) experiencing ovulatory cycles proven by a single progesterone level of 3 ng/mL or greater during the luteal phase of the screening cycle. Exclusion Criteria: * Pregnant, seeking pregnancy, or breastfeeding * Known allergy to study medication * Recent use of hormonal contraception * Irregular periods (\<21 days or \>35 day cycles) * Routine use of nonsteroidal anti-inflammatory drugs * Metabolic disorders * Smoking * Any condition that would preclude the provision of informed consent * Using drugs (within 2 weeks of study enrollment) known to interfere with the metabolism of UPA as well as drugs known to be CYP3A4 inducers, inhibitors, or CYP3A drug substrates