 - The treatment of addiction often hinges on preventing relapse into drug-using behaviors, which occurs at high rates even after prolonged abstinence. Some methadone patients continue to abuse cocaine and heroin during treatment, even with extensive psychosocial services. More research is needed to look at the results from earlier studies of continued drug use during methadone treatment, focusing on the results of fixed vs. flexible doses of methadone to reduce the likelihood of continued drug use and the role of monetary vouchers as an incentive to continue abstinence from illicit substances. 