The purpose of this study is to determine if there will be a significantly higher incidence of a bowel movement with methylnaltrexone vs. placebo within 4 hours +- 45 minutes with decreased need for rescue medications in the intensive care unit in patients with opioid-induced constipation. Patients will also be managed with an aggressive bowel management protocol.

Inclusion Criteria:

• 18 years of age or older in the ICU
• Opioids for analgesia for at least 24 hours.
• Opioid-induced constipation with no bowel movement within the last 72 hours.
• Women of childbearing potential had negative pregnancy tests.

Exclusion Criteria:

• Contraindication to use of the GI tract
• Diarrhea on admission
• Bowel surgery within 8 weeks of admission
• Ileostomy or colostomy
• Not expected to live or stay more than 3 days in the intensive care unit
• Constipation that was not primarily caused by opioids (as determined by the investigator)
• No opioid use in the last 24 hours,
• Mechanical gastrointestinal obstruction
• An indwelling peritoneal catheter
• Clinically active diverticular disease
• Fecal impaction
• Acute surgical abdomen
• History of Crohn's disease or ulcerative colitis
• On Palliative care
• Less than 18 years old
• Bowel movement in last 72 hours.