Safety Study to Assess Opiate Withdrawal Signs and Symptoms in Opioid Dependent Patients

Inclusion Criteria: * Chronic moderate to severe non-cancer pain that has been treated with opioid analgesics for at least three months (with stabilized pain control and stabilized dose for 28 days prior to enrollment). * Receiving an opioid dose equivalent to 20 mg - 120 mg morphine once or twice daily. * Patient displays signs and symptoms of withdrawal (i.e., COWS score ≥5) following naloxone administration during the Naloxone Challenge. If female and able to become pregnant, must use an approved method of birth control. * Excluding the chronic moderate to severe non-cancer pain, the patient is judged by the Investigator to be in generally good health at screening based upon the results of a medical history, physical examination, laboratory profile, and 12 lead electrocardiogram (ECG). Exclusion Criteria: * Female who is pregnant or breastfeeding. * Patient has a known allergy or history of significant adverse reaction to morphine, other opioids, naltrexone, acetaminophen, or related compounds. * Patient is receiving systemic chemotherapy, has an active malignancy of any type, or has been diagnosed with cancer within the 5 years prior to screening (excluding squamous or basal cell carcinoma of the skin). * History of, or ongoing, alcohol or drug abuse. * Patient has made a donation of blood or has had a significant blood loss within 30 days prior to screening.