This study tests how well a drug called elagolix works for women with endometriosis, a painful condition where tissue similar to the inside of the uterus grows outside it. Women with endometriosis often have painful periods, known as dysmenorrhea. The study compares elagolix combined with birth control pills to a placebo (a pill with no medicine) to see if it helps with pain relief.

Participants will be divided into three groups and have a 1 in 3 chance of getting the placebo. About 800 women in different locations, including Puerto Rico, will take part. For the first 3 months, participants will receive either elagolix or placebo with birth control pills. From month 4 to 18, everyone will get elagolix with birth control pills.

• The study lasts 18 months with regular hospital or clinic visits.
• Participants must use non-hormonal birth control methods during certain study periods.
• Women who are pregnant, breastfeeding, or planning pregnancy cannot join.

Medical check-ups, blood tests, and questionnaires will monitor the treatment effects and any side effects.