A Study to Evaluate the Efficacy and Safety of Gefapixant (MK-7264) in Women With Endometriosis-Related Pain (MK-7264-034)

Inclusion Criteria: * has been surgically (laparoscopy or laparotomy) diagnosed with endometriosis. * has cyclic AND non-cyclic, moderate to severe endometriosis-related pelvic pain (overall pelvic pain score ≥5 using a 0-10 NRS, with 0 representing no pain and 10 representing extremely severe pain). * has had spontaneous menstrual cycles before Visit 1. * has body mass index (BMI) between 18 kg/m\^2 to 40 kg/m\^2 at Visit 1. * is not pregnant, not breastfeeding, and agrees to follow the contraceptive guidance. * must agree to switch from her usual analgesic medication to only that which is permitted in the study. Exclusion Criteria: * history of hysterectomy and/or bilateral oophorectomy. * has undiagnosed vaginal bleeding. * has chronic, non-pelvic pain not caused by endometriosis that requires chronic analgesic. * has a clinically significant gynecologic condition identified in the screening evaluation. * has a history of anaphylaxis or cutaneous adverse drug reaction (with or without systemic symptoms) to sulfonamide antibiotics or other sulfonamide-containing drugs. * has a known allergy/sensitivity or contraindication to gefapixant or its excipients. * has an allergy/sensitivity/intolerance to naproxen sodium (rescue medication) or any contraindication to its use, or has experienced asthma, urticaria, or allergic-type reactions after taking aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs). * has a history of endometriosis-related pain that was non-respo…