This study will evaluate the safety and efficacy of methylnaltrexone administered as subcutaneous injections in subjects who have opioid-induced constipation and an advanced illness. The hypothesis is that methylnaltrexone will be safe and effective in relieving opioid-induced constipation in these subjects.

Inclusion Criteria:

• Is an adult 18 years of age or older
• Has a diagnosis of advanced illness (ie, a terminal illness such as incurable cancer or other end-stage disease)
• Has a life expectancy of at least 1 month.
• Is receiving opioids on a regular schedule (not just as-needed or rescue doses) for the control of pain or discomfort for at least 2 weeks before the first dose of study drug.
• Has constipation that is caused by opioid medications.

Exclusion Criteria:

• Has a known or suspected allergy to methylnaltrexone or other similar compounds (e.g. naltrexone or naloxone).
• Has a known or suspected mechanical gastrointestinal obstruction.
• Has any potential nonopioid cause of bowel dysfunction that might be a major contributor to the constipation.
• Has any other clinically important abnormalities as determined by the investigator that may interfere with his or her participation in or compliance with the study.
• Receiving opioid antagonist or partial antagonist products.