This paper reviews preclinical research which demonstrates the involvement of the opioid system in the reinforcing effects of alcohol, and the effects of naltrexone, a long-acting, nonselective opioid receptor antagonist to reduce alcohol intake. Naltrexone (50 mg/day) may prevent the return to drinking by blocking the pleasurable effects or "high" associated with alcohol drinking, and relapse rates were reduced. The most common adverse effects reported include nausea and vomiting and it does not appear to be hepatotoxic in dosages recommend. Thus, naltrexone appears to offer significant therapeutic benefits, when used with behavioural treatment for alcohol dependent patients.