Studies have shown that the comorbidity of attention-deficit/hyperactivity disorder (ADHD) and substance-use disorder (SUD) in adolescents ranges from 16% (population studies) to 25–40% (clinical populations) (1
). The presence of ADHD may affect the course of adolescent substance abuse in several ways, including increased likelihood of an earlier age of onset, longer duration of SUD, and progression from alcohol abuse to another drug-use disorder. Individuals with ADHD are also at greater risk for treatment failure as their disruptive behaviors interfere with treatment access, compliance, and response.
While there is a strong relationship between ADHD and SUD, less is known about the relationship of ADHD subtypes (i.e., inattentive, combined, and hyperactive-impulsive) and SUD. The inattentive subtype is characterized by inattention, distractibility, disorganization, forgetfulness, and lethargy, while the hyperactive-impulsive subtype is characterized by hyperactivity and/or impulsiveness (e.g., interrupting, difficulty waiting turn, fidgetiness). The combined subtype, most commonly represented in studies of ADHD, includes both inattentive and hyperactive-impulsive symptoms. There are few studies investigating the hyperactive-impulsive subtype, which is much less prevalent, and there are significant concerns regarding its validity (2
). However, there is a burgeoning literature suggesting different phenotypic profiles for the ADHD inattentive and combined subtypes including different patterns of psychiatric comorbidity (e.g., (3
)), gender ratios (e.g., (4
)), and response to medication (e.g., (5
)). For example, the inattentive subtype has been shown to respond to lower doses of stimulant medication than the combined subtype (5
). These two subtypes have also been characterized by different neuropsychological profiles (e.g., (2
)), with the combined subtype showing a disinhibited/impulsive response style, and the inattentive subtype showing a sluggish cognitive tempo. Recent literature suggests a relationship between cognitive functioning and SUD treatment outcomes (6
). Impaired executive functioning was found to be more predictive of tobacco use than behavioral activity (7
), while better cognitive functioning has been associated with greater treatment gains (8
). Therefore, there may be a different pattern of substance use and/or response to treatment between the inattentive and combined subtypes.
The literature investigating the association between ADHD subtypes and SUDs is mixed. Some studies do not report a significant relationship (e.g., (3
)), while others have suggested that hyperactive/impulsive symptoms are more associated with risk for SUDs (e.g., (9
)) than inattentive symptoms (10
). A prospective study showed that hyperactivity/impulsivity predicted initiation of all types of substance use and that relationships between inattention and substance outcomes disappeared when hyperactivity/impulsivity and conduct disorder were controlled (except nicotine dependence) (9
). Furthermore, a study in adults with ADHD reported that the combined subtype had a higher incidence of lifetime SUDs than the inattentive subtype (11
), suggesting greater contribution of hyperactive/impulsive symptoms. Other studies report differences between the subtypes and types of SUDs. For example, hyperactivity/impulsivity has been associated with earlier smoking and illicit drug use (12
), alcohol use (13
), dependence (14
), and cocaine dependence (15
). In contrast, inattention has been associated with early illicit drug use, frequency and recency of alcohol and marijuana use, and heavier cigarette use (12
), tobacco (13
) and marijuana use, and nicotine dependence (16
). Thus, the findings are inconclusive.
There is a dearth of research investigating the relationship between ADHD and SUD in regard to treatment outcomes, with even less known about the relationship of ADHD subtype and substance treatment outcomes. Arguably, individuals with the combined or hyperactive-impulsive types, types characterized by poor judgment and impulsivity, may be more vulnerable to treatment drop-out and relapse. One study reported that the inattentive subtype was more amenable to treatment for nicotine addiction while the hyperactive-impulsive subtype had lower abstinence rates (14
). No other literature was found investigating whether ADHD subtypes and SUD are associated with differential treatment response for ADHD or substance-use outcomes and/or describing demographic/clinical characteristics of adolescents with co-occurring ADHD-SUD. Thus, the relevance of ADHD subtypes as predictors of SUD or ADHD treatment outcomes remains unexplored.
A study performed in the Clinical Trials Network (CTN) funded by the National Institute on Drug Abuse entitled Osmotic-Release Methylphenidate (OROS-MPH) for ADHD in Adolescents with SUD (CTN0028) evaluated the efficacy of OROS-MPH, relative to placebo, in treating ADHD and decreasing substance use in adolescents with ADHD-SUD. All participants received weekly cognitive behavioral therapy (CBT) focusing on SUD. CBT consisted of approximately one 60-minute session per week during study weeks 1–16 and focused on the treatment of the adolescent’s SUD. The CBT manual utilized was based a manual that was developmentally adapted for adolescents from a standard, published, empirically supported adult CBT manual (17
). It was not adapted for ADHD. Motivational enhancement techniques were used in conjunction with behavioral and cognitive behavioral techniques to help adolescents reduce their drug use by improving coping strategies and problem-solving and decision-making skills. Results of the primary study showed that both medication and placebo groups had significant reductions in ADHD and substance-use outcomes and will be reported elsewhere (18
). The study provides an excellent opportunity to investigate the ADHD subtypes in a comorbid ADHD-SUD population.
The current report is designed to answer questions from CTN0028 data: (1) Are there baseline differences in the demographic and clinical characteristics, as well as patterns of substance use, between the inattentive and combined ADHD subtypes? (2) Are the subtypes associated with differential treatment response (medication response, substance-use outcomes)? Given the inconclusive findings regarding the subtypes and SUD and the scant previous literature on treatment response as a function of subtype, we did not have specific hypotheses regarding the direction of findings.