Naltrexone is used in opioid dependence to:

Naltrexone a long acting opioid antagonist competitively block the opioid receptor and blocks the effects of opioids. Regular dosing of naltrexone prevents patients from experiencing the effect of opioids thereby preventing relapse. Opioid antagonists are used in the treatment of opioid dependence in 3 ways: To reverse an opioid overdose To prevent relapse by blocking opioids from producing their effect at opioid receptors To identify those with opioid dependence by inducing withdrawal symptoms to provide evidence of opioid dependence. Ref: Cambridge Textbook of Effective Treatments in Psychiatry by Peter Tyrer, Page 386