Ulipristal for Endometriosis-related Pelvic Pain

Inclusion Criteria: * Diagnosis of endometriosis with surgical confirmation via laparoscopy performed within 3 years prior to study enrollment * English-speaking reproductive-age women (18-50yo) with regular, cyclical menses * Treatment-refractory endometriosis, as defined by persistent or recurrent pelvic pain following one or more treatments involving surgery and/or hormonal treatment * Dysmenorrhea and chronic pelvic pain for a least one week out of the month during past 5 months or longer * Endometrial biopsy before and after intervention * Adequate organ and marrow function as defined below: * leukocytes ≥ 3,000/microliter * absolute neutrophil count ≥ 1,500/microliter * platelets ≥ 100,000/microliter * total bilirubin within normal institutional limits * Liver function tests ≤ 2.5 X institutional upper limit of normal * creatinine within normal institutional limits * Women of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for 90 days following completion of therapy. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. * A female of child-bearing potential is any woman (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: * Has not undergone a hysterectomy or bilateral oophorectomy; or * Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months). * Ability to understand and the willingness to sign a written informed consent. Exclusion Criteria: * Undiagnosed vaginal bleeding * Abnormal results from endometrial biopsy * Presence of ovarian cysts ≥ 3 cm * Pregnancy * Refusal to adhere the birth control strategy of the study (use of a combination of a condom and a vaginal sponge during each intercourse while on ulipristal, plus one month before the onset and one month after the completion of ulipristal therapy) * Women taking digoxin, dabigatran or etexilate due to interference of ulipristal acetate with the concentration of these substances bound by plasma proteins. * Women taking moderate to potent inducers of hepatic metabolism (e.g. erythromycin, ketoconazole, ritonavir, nefazodone). * History of allergic reactions attributed to compounds of similar chemical or biologic composition to Ulipristal. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active pulmonary, cardiac, renal, hepatic or thromboembolic disease, infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.