Additional Study Information: This research study involves the use of an investigational drug (a drug that is not approved by the FDA) in premenopausal woman with moderate to severe pain associated with endometriosis. In addition to the investigational drug, you may also receive a small amount of estrogen/progestin medication to decrease some of the side effects from the investigational drug, called add-back therapy. The add-back therapy is being studied with the investigational drug to see if it will reduce the occurrence of symptoms associated with low estradiol levels such as hot flashes, and to prevent potential bone loss which may occur in women who take the investigational drug. Therefore, the use of the add-back therapy is investigational (experimental) for the purposes of this study.