An Efficacy and Safety Study of Elagolix (NBI-56418) in Women With Endometriosis

* Be female, aged 18 to 49 years, inclusive * Have moderate to severe pelvic pain due to endometriosis * Have been surgically (laparoscopy) diagnosed with endometriosis within the last 8 years and have recurrent or persistent endometriosis symptoms * Have regular menstrual cycle * Have a body mass index (BMI) of 18 to 36 kg/m², inclusive * Agree to use two forms of non-hormonal contraception during the study

* Are currently receiving gonadotropin-releasing hormone (GnRH) agonist or GnRH antagonist or have received any of these agents within 6 months of the start of screening * Are currently receiving subcutaneous medroxyprogesterone acetate (DMPA-SC) or intramuscular medroxyprogesterone acetate (DMPA-IM) or have received any of these agents within 3 months of the start of screening * Are currently using hormonal contraception or other forms of hormonal therapy or received such treatment within the last month * Have had surgery for endometriosis within the last month * Have had a hysterectomy or bilateral oophorectomy * Are using systemic steroids on a chronic or regular basis within 3 months * Have uterine fibroids ≥ 3 cm in diameter * Have pelvic pain that is not caused by endometriosis * Have unstable medical condition or chronic disease * Have been pregnant within the last six months * Currently breast feeding