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.

- 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-responsive to treatment with
combined hormonal contraceptives (CHCs), gonadotropin-releasing hormone (GnRH)
antagonists, GnRH agonists, progestins, or aromatase inhibitors.

- has a positive urine pregnancy test at any time before randomization.

- has required more than 2 weeks of continuous use of narcotics for treatment of
endometriosis-related pain within 6 months of Visit 1.