A Study of Oral Methylnaltrexone (MNTX) for the Treatment of Opioid-Induced Constipation (OIC) in Participants With Chronic, Non-Malignant Pain

Key Inclusion Criteria: 1. History of chronic non-malignant pain (originating from a non-malignant source) with condition(s) underlying the chronic pain of greater than or equal to (≥) 2 months' duration before the screening visit. 2. Taking oral, transdermal, intravenous (IV), or subcutaneous (SC) opioids for chronic non-malignant pain for ≥1 month. 3. No known history of chronic constipation prior to the initiation of opioid therapy. 4. Currently taking laxative therapy for ≥30 days and willing to discontinue all laxative therapy at the start of screening period and use only study-permitted rescue laxatives throughout the screening and double-blind treatment periods.. Key Exclusion Criteria: 1. Prior treatment with oral MNTX. 2. Prior treatment with SC MNTX within 30 days of screening. 3. Women who are pregnant, breastfeeding, or plan to become pregnant during the study. 4. Fecal incontinence, rectal prolapse, fecal ostomy or other clinically significant gastrointestinal disorders such as inflammatory bowel disease or clinically significant irritable bowel syndrome that would have made bowel movement assessment inaccurate. 5. Current treatment with partial opioid agonists (for example; buprenorphine) or combination agonists/antagonists.