This is an open label compassionate use study of subcutaneously administered methylnaltrexone (MNTX) in participants with advance medical illness and opioid-induced constipation.

Inclusion Criteria:

• Advanced medical illness (that is; terminal illness, such as incurable cancer or end stage aqcuired immunodeficiency syndrome [AIDS]) with a life expectancy of one to six months.
• On an opioid regimen for the control of pain/discomfort for at least seven days.
• Has opioid-induced constipation.
• Stable vital signs and systolic blood pressure greater than or equal to (>=) 85 millimeters of mercury (mmHg), and diastolic blood pressure >=45 mmHg (Supine or sitting).
• Females of childbearing potential must have a negative pregnancy test (serum or urine).
• On a laxative regimen (for example, stool softener and SENNA or equivalent) for at least 3 days prior to treatment. Participants who have discontinued laxatives due to intolerability or lack of efficacy are also eligible.

Exclusion Criteria:

• Participants with known hypersensitivity to methylnaltrexone, naltrexone or naloxone.
• Participants who received any investigational new drug (experimental) except for methylnaltrexone in the previous 30 days.
• Participants who are constipated with a disease process suggestive of gastrointestinal obstruction, impaction or diagnosed with a current abdominal pathologic process which may represent a non-opioid cause of bowel dysfunction.
• Participants who are constipated and have active, clinically significant diverticulitis.
• Participants with a surgically acute abdomen.
• Participants being treated with opioids for diarrhea, dyspnea, cough, pulmonary edema, or congestive heart failure.
• Individuals with a known drug addiction.
• Females who are pregnant or nursing.

All enrolled participants who provided a signed and dated written informed consent form (ICF).

From start of treatment until end of study (up to maximum 3.4 years)

Safety evaluable participants included all enrolled participants who received at least one dose of study drug.