Alvimopan is a gastroprokinetic peripheral mu-opioid receptor antagonist. As a drug, Alvimopan is given to patients undergoing bowel surgery to protect the bowel from the effects of opioid medications that are used to treat pain, blocking the undesirable GI side-effects of the opioid agonists without affecting analgesia or causing withdrawal. Alvimopan does not cross the blood-brain barrier, competitively binding to gastrointestinal tract mu-opioid receptors.