Naltrexone is used in opioid addiction because -

Naltrexone can be used orally to assist in the rehabilitation of ex-opioid abusers who are fully withdrawn. However, It should be kept in mind that naltrexone should be used only after full withdrawal of opioid, otherwise acute withdrawal syndrome will be precipitated as naltrexone antagonizes the action of opioids on opioid receptors. Naltrexone prevents relapse by discouraging substance seeking behavior → If a patient, who is on naltrexone maintenance therapy, takes on an opioid, there is no 'kicks' or 'euphoria' as opioid receptors are already blocked.