Endometriosis is a painful disorder of the uterus affecting 6-10% of women of childbearing age. Endometriosis affects daily activities, social relationships, sexuality and sexual activity, and mental health. This study will evaluate how well elagolix in combination with combined oral contraceptives (COC) works within the body and/or how safe it is compared to placebo (does not contain treatment drug). This study will assess the dysmenorrhea (painful periods) response in participants with endometriosis and associated pain. Elagolix is an approved drug for the management of moderate to severe pain associated with endometriosis. Participants are randomly put in 1 of 3 groups, called treatment arms. Each group receives a different treatment. There is a 1 in 3 chance that participants will be assigned to placebo. Adult female participants who still have periods with a diagnosis of endometriosis will be enrolled. Around 800 participants will be enrolled in the study at multiple sites in the United States, including Puerto Rico. Participants will receive oral elagolix or placebo tablets in combination with combined oral contraceptive (COC) or placebo capsules for 3 months. All the participants will receive elagolix tablets in combination with COC tablets from Month 4 through Month 18. There will be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the course of the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.