Naltrexone decreases the craving for ethanol and is approved for adjunctive use in alcohol dependency programs. Unlike the older drugs, two new antagonists, methylnaltrexone and alvimopan, do not cross the blood-brain barrier. These agents block adverse effects of strong opioids on peripheral μ receptors, including those in the gastrointestinal tract responsible for constipation, with minimal effects on analgesic actions and without precipitating an abstinence syndrome.