Inclusion Criteria:  Experimental Group (initial screen):  Age 18 years or older Inpatient who will undergo a 1-2 level ALIF procedure with Dr. Richard Meyrat Are not pregnant or lactating Does not have a history of drug abuse within the past 2 years Does not have a gastrointestinal ostomy  Experimental Group (post-operative OIC screen/rescreen)  Actively constipated (i.e., no BM in last 48 hours, difficult having a spontaneous BM, or inability to have a spontaneous BM) Receiving mu-agonist opioids Expected to require daily opioid analgesics for at least 7 days following enrollment Have not received or discontinued use of any laxatives, enemas and/or promotility agents within 48 hours of enrollment and the first dose of RELISTOR. Stool softeners are permitted. Does not have a known hypersensitivity to methylnaltrexone, naltrexone, or naloxone  Observational Group (initial screen):  Age 18 years or older Inpatient who will undergo 1-2 level ALIF procedure with Dr. Graham Are not pregnant or lactating Does not have a history of drug abuse within the past 2 years Does not have a gastrointestinal ostomy  Observational Group (post-operative OIC screen/rescreen):  Actively constipated (i.e. no BM in last 48 hours, difficulty having a spontaneous BM, or inability to have a spontaneous BM) Receiving mu-agonist opioids Expected to require daily opioid analgesics for at least 7 days following enrollment  Exclusion Criteria:  Experimental Group (post-operative OIC screen/rescreen):  Exclude patients with:  Fecal impaction Mechanical bowel obstruction Constipation not attributed to post-procedural opioid use Calculated creatinine clearance less than 50 mL/min Corrected QT interval greater than 500 msec. on a 12-lead screening electrocardiogram  Observational Group (post-operative OIC screen/rescreen):  Exclude patients with:  Fecal impaction Mechanical bowel obstruction Constipation not attributed to post-procedural opioid use