In this randomized trial of voucher-based CM and MET/CBT for young marijuana-dependent individuals referred to treatment by the criminal justice system, there were consistent effects favoring CM for treatment retention and marijuana use outcomes, both of which were specifically targeted by the CM system used here. There were few significant main effects for MET/CBT over DC for the full sample during the active phase of treatment. However, there were several significant interaction effects suggesting that MET/CBT combined with CM was associated with better outcomes than MET/CBT without CM, DC plus CM, and that those three treatments were significantly more effective than DC without CM. Finally, there was evidence of continuing improvement during the 6-month follow-up for those assigned to the MET/CBT condition.
This study extends the literature supporting the efficacy of CM interventions in a range of substance-using populations in several ways. First, the study underscores the feasibility of CM procedures with marijuana-dependent populations. Given that the effectiveness of CM has been associated with the exposure of individuals to reinforcers (Petry, 2000
) as well as rapid and accurate detection of target behaviors (in this case, the initiation of abstinence; Budney & Higgins, 1998
; Higgins et al., 1991
), the comparatively extended half-life of marijuana could potentially complicate use of CM procedures with this population, given that marijuana may be detectible in urine up to 3 weeks after the initiation of abstinence (Hawks & Chiang, 1986
; Schwartz, 1988
). During the 8-week active phase of treatment, 46% of participants submitted at least one marijuana-free urine sample, and more than half of the participants assigned to the CM condition (37/67) earned at least one voucher for urine-verified abstinence during the trial. Most participants were able to produce a marijuana-free urine specimen approximately 10 days after their last reported use of marijuana; moreover, the level of agreement between participants’ self-reports of marijuana use and urinalysis results was consistent with those reported among previous clinical trials of other types of drug users (Zanis, McLellan, & Randall, 1994
). This is notable given that the participants, all of whom were referred by the criminal justice system, faced both significant negative consequences (i.e., the threat of incarceration for continued illegal behavior and drug use) and positive consequences for abstinence (i.e., earned goods and services if assigned to CM).
Second, the strategy of reinforcing both session attendance (so that most participants could have some exposure to the reinforcers simply by attending sessions) and abstinence (with a comparatively large incentive for the first marijuana-free urine specimen) appeared to be effective for this population, particularly when delivered in combination with MET/CBT. It should be noted that significant effects favoring CM were found primarily on those outcomes that were reinforced (session attendance and marijuana-free urine specimens) and did not appear to generalize to other indicators of outcome (e.g., ASI composite scores addressing other psychosocial outcomes). On the other hand, the 8-week trial was of comparatively short duration, and more time may have been necessary for treatment, or the initiation of abstinence, to have meaningful effects on other problem areas (e.g., legal and employment functioning).
Finally, although several studies have evaluated combinations of CM and CBT to enhance outcome (Budney & Higgins, 1998
; Epstein, Hawkins, Covi, Umbricht, & Preston, 2003
; Rawson et al., 2002
), this is the first study to our knowledge to evaluate systematically whether outcomes could be enhanced through a combination of CM with different types of well-defined behavioral therapies, in this case MET/CBT versus a manualized DC condition that had strong empirical support (Carroll, Nich, Ball, McCance-Katz, & Rounsaville, 1998
; Crits-Christoph et al., 1999
). The combination of CM and MET/CBT was significantly more effective than any of the other conditions (MET/CBT without CM, DC plus CM, and DC without CM) in terms of several important outcome measures, including retention in treatment, number of consecutive marijuana-free urine specimens submitted, and the likelihood of submitting a marijuana-free urine specimen across the treatment period. Even for those outcome indicators for which no statistically significant differences were found, the combination of MET/CBT and CM was consistently associated with the best outcomes overall. This thus supports our hypothesis that CM would be more effective when combined with MET/CBT. It is possible that CM may have enhanced MET/CBT by allowing more participants to sample the benefits of abstinence, to think more clearly about treatment goals, and to “own” the decision to reduce marijuana use, particularly when compared with DC, in which abstinence was not discussed as a choice. Moreover, although it was conceivable that CM (with its emphasis on providing extrinsic motivation for change via provision of incentives for abstinence and attendance) might work against MET/CBT (with its emphasis on enhancing intrinsic motivation; Deci, Koestner, & Ryan, 1999
), there was little evidence, at least from the retention and marijuana use data, that this was the case in this study. Not only were outcomes consistently better in the MET/CBT plus CM combination, but in several cases they were significantly superior to the DC plus CM combination, suggesting that further work on identifying the most efficacious combinations of CM and various behavioral therapies may be promising.
In contrast to the strong and consistent effects for CM, there were comparatively few indications of a significant main effect for MET/CBT compared with DC alone during the active phase of treatment, with the exception of the ASI legal composite score. There was, however, evidence of continuing improvement for participants assigned to the MET/CBT condition during follow-up. Because the MET/CBT condition used here included significant attention to skills-building approaches, this finding is consistent with previous findings of durable, continuing improvement with cognitive–behavioral approaches (Carroll et al., 1994
; Hollon, 2003
; Rawson et al., 2002
). Moreover, the MET/CBT plus CM combination consistently produced the best outcomes overall and outcomes that were markedly more positive compared with the condition intended to approximate standard treatment at the performance site (DC without CM). In fact, the 39% completion rate for the DC without CM condition is consistent with the typical retention rates at the clinic for this population (Sinha et al., 2003
There are several limitations to this study. First, the duration of treatment was comparatively brief (8 weeks); a longer course of treatment may have allowed more participants to become abstinent or for other benefits of the study treatments to emerge. Second, because the MET/CBT treatment included elements of both motivation interviewing and cognitive–behavioral therapy, it is not possible to ascribe the treatment effects seen here to particular elements of either approach. Third, urine specimens were collected only once per week; more frequent collection would not necessarily have detected other instances of marijuana use not reported by the participants, given marijuana’s comparatively long half-life.
There were several other findings of note. These data support and extend those reported by Moore and Budney (2002)
, suggesting that an early marijuana-positive urine sample was strongly associated with outcome, thus linking the treatment outcome literature for marijuana dependence to that of cocaine. On the other hand, whether the first urine specimen was positive for marijuana was not significantly related to retention or treatment completion; this may reflect both the comparatively brief duration of the 8-week treatment as well as the marked effects of the MET/CBT plus CM combination on treatment completion.
Finally, the study sample was noteworthy in several respects, in that the participants were primarily young African American men with an average of five arrests by the age of 21, 43% met diagnostic criteria for antisocial personality disorder, and most were unemployed and had not completed high school. Although referred to treatment by the legal system and hence likely to incur significant consequences for continued drug use, none had completely stopped marijuana use at the time of their application for treatment. Thus, it is promising that (a) effect sizes for CM and for the MET/CBT plus CM condition were in the moderate range (.25–.47) for retention and marijuana use outcomes, and (b) the MET/CBT plus CM condition doubled the rate of individuals who completed treatment and submitted at least one marijuana-free urine specimen during treatment compared with the DC without CM condition (the latter approximated the rate associated with standard treatment at the clinic [46% vs. 21%]). Furthermore, previous clinic data on individuals from this population underlined that their retention in treatment was strikingly poor, even in the context of the implied consequences imposed by the criminal justice system (Sinha et al., 2003
). Hence, the fact that more than half of the sample completed treatment and the combined MET/CBT plus CM approach evaluated here was associated with retention of 70% of participants through the end of treatment is of great potential significance regarding treatment effectiveness for this very challenging population.
This study also underscores the difficulty of applying standard definitions of clinical significance (Jacobson, Roberts, Berns, & McGlinchey, 1999
; Jacobson & Truax, 1991
; Kendall, Marrs-Garcia, Nath, & Sheldrick, 1999
) to treatment studies with substance-using populations; these problems are heightened with nonnormative behaviors such as illegal drug use. Rather than well-normed psychological assessments with established psychometric properties (i.e., the Beck Depression Inventory), treatment outcome research in substance abuse relies on indices such as frequency and intensity of substance use, as determined through self-report or biological measures. These indices tend to be highly variable; thus, determining the proportion who met cutoffs such as change of 2 standard deviations (Jacobson & Truax, 1991
), calculation of a Reliable Change Index (Jacobson et al., 1999
), no longer meeting diagnostic criteria for substance dependence disorder (Kazdin, 1999
), or even comparison with population norms (Kendall et al., 1999
) would, in most studies, as it would have in this one, require a standard of abstinence from marijuana and other substances. The multiple problems with highly insensitive measures to evaluate complete abstinence as an outcome are well known (Babor et al., 1994
; McLellan, McKay, Forman, Cacciola, & Kemp, 2005
), and thus the measure used here (demonstration of some abstinence plus treatment retention) is less than ideal. Further work on this area is needed to facilitate outcome comparisons across different studies.