than 20 years in the treatment of opiate dependence (see Greenstein et al., 1981; O'Brien et al., 1984). It is an orally administered opiate antagonist that blocks actions of externally administered opiates such as heroin by competitive binding to opiate receptors. It has been particularly effective as an adjunct to probation in opiate addicted federal probationers (see Cornish et al., 1997). More recently, naltrexone (marketed under the trade name Revia®) has been found to be effective in the treatment of alcohol dependence (O'Malley et al., 1992; Volpicelli et al., 1992). Naltrexone at 50mg/day has been approved by the FDA for use with alcohol dependent patients since independent studies have shown it to be a safe, effective pharmacological adjunct for reducing heavy alcohol use among alcohol dependent patients. Its mechanism of action appears to be the blocking of at least some of the "high" produced by alcohol consumption, again through competitive binding with the mu opiate receptors (O'Malley et al., 1992; Volpicelli et al., 1992).

With regard to other medications designed to block the effects of an abused drug, disulfiram (Antabuse ®) has been used the longest and most pervasively in the treatment of alcohol dependence (see Fuller et al., 1986). However, disulfiram seems to be most effective under certain conditions, such as when the patient contracts to having a significant other witness him or her take the medication each day. More recently, European researchers have found encouraging results with acamprosate as a treatment for alcoholism (Ladewig et al., 1993; Lhuintre et al., 1990). While acamprosate acts on different receptor systems than naltrexone, the clinical results are remarkably similar (Anton, 1995; Ladewig et al., 1993; Lhuintre et al., 1990). Alcohol dependent patients who take acamprosate have shown 30% greater posttreatment abstinence rates at six-month follow-up than those randomly assigned to placebo. Further, those who have returned to drinking while taking acamprosate report less heavy drinking (greater than five drinks per day) than those who returned to drinking while prescribed placebo (Anton, 1995). While both of these medications can be used for extended periods, in practice they are generally prescribed for about one to three months as part of a more general rehabilitation program that includes behavioral change strategies (see review by Anton, 1995).

There have been many agents tried as blocking agents in the treatment of cocaine dependence and while this literature is quite large, it has been disappointing (see Institute of Medicine, 1995; O'Brien, 1996; O'Brien and McKay, in press). At this writing, there is no convincing evidence that any of the various types of cocaine blocking agents are truly effective for even brief periods of time or for even a significant minority of affected patients. Research continues in this important area and there have been indications of a potentially successful "vaccine" that may be able to immediately metabolize and inactivate active metabolites of cocaine (see Fox, 1997). This

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