2 49 The effectiveness of Take it Personal! on substance use this decrease was only reflected in an overall decrease of severity of alcohol use and not in a decrease of severity of cannabis and illicit drug use. Indicators of substance use severity are—in addition the frequency of substance use—symptoms of dependence and problems related to use (Babor et al., 2001, Berman et al., 2003). Most participants in our study participated in Take it Personal! for help with their problematic cannabis use (although often in combination with alcohol or illicit drug use). For this reason, it can be expected that problems related to cannabis are more persistent and more difficult to change, on average, than problems related to alcohol use in this particular group. A decrease of drug use dependence symptoms and problems may follow after a longer period of decreased substance use frequency. The current study has some limitations. First, participants were assigned to the intervention and control condition based on treatment center. The absence of participant randomization between conditions may have influenced the results. Secondly, in our study the personality profiles sensation-seeking and impulsive behavior were overrepresented. Adolescents in our sample often obtain high scores on more than one personality profile, and in most cases substance use was attributed to the externalizing profiles, as internalizing profiles might stand out less. Moreover, trainers speculated that adolescents with anxiety sensitivity- or negative-thinking profiles have less motivation to participate in a prevention program for substance use and an effectiveness study. There are different reasons why adolescents with anxiety sensitivity or negative thinking have less motivation to participate. Research suggests that the presence of anxiety sensitivity or depression affects motivation in general, and more specifically motivation and adherence to interventions due to a variety of client-related factors such as illness beliefs and attitudes (Lingam & Scott, 2002). In a similar vein, adolescents with negative and anxious personalities may find it harder to be motivated to participate in our study. Thirdly, in this study we only investigated short-term intervention effects. Although the initial plan was to also conduct long-term assessments of the effects, for practical reasons (e.g. clients leaving the treatment facility) this appeared to be impossible. Therefore, it was not possible to draw conclusions concerning the long-term effects of the intervention. Future studies should focus on long-term effects of Take it Personal! This study shows that cannabis use is the most significant problem in adolescents with a mild intellectual disability, and clinicians from several treatment centers confirmed that cannabis contributes to the greatest problems in the daily life of our target population. Most participants in our study were enrolled into the prevention program for problems related to cannabis use.
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