Objectives: Review and summarize the mechanism of action of methylnaltrexone bromide (methylnaltrexone) and its effectiveness in the treatment of opioid-induced constipation. Data Source: A multi-database search was conducted using PubMed and MEDLINE databases, in addition to electronic links to related articles and references. Background: Opioids are effective medications for the management of moderate to severe pain, but they are associated with a number of side effects, especially within the gastrointestinal system. Constipation is a very common adverse reaction in patients with late-stage, adverse illness, who require long term administration of opioids on a chronic basis to help alleviate pain. In April 2008, the Food and Drug Administration approved the use of methylnaltrexone, a quaternary derivative of naltrexone which does not cross the blood brain barrier, for the management of patients with opioid-induced constipation. Methylnaltrexone acts as a selective peripheral Mu-receptor antagonist, without affecting the effects of opioids on central analgesia. Conclusions: Studies have been shown that methylnaltrexone can be used safely in the treatment of opioid-induced constipation without either interfering with opioid effects on central anesthesia or precipitating opioid withdrawal.