The sample was composed of 93 young adults, 43 without ADHD (no ADHD) and 50 with ADHD, who were part of the Spanish sample of the Multicenter ADHD Genetics (IMAGE) study. When the ADHD participants were first evaluated between 2003 and 2006, were between 5 and 16 years old and presented a clinical diagnosis of ADHD combined-subtype, according the international diagnostic criteria for this subtype from the revised fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR; American Psychiatric Association, APA, 2000) (see description of the sample in Müller et al., 2011). Fifty of 81 subjects from original sample of IMAGE study participated in the current follow-up evaluation, which took place on average 10.1 years later, between 2013 and 2015. Fourteen subjects (17%) did not participate because of loss of their localization (changes in the family address or contact telephone), 7 participants declined to attend to the evaluations (9%), and 10 participants were less than 17 years old (12%).

The ADHD group had between 17 and 24 years (M=18.7, SD=1.4) and had an IQ between the normal range (M=104.21, SD=15.41). Additionally, 65% of these participants according to the subjects themselves and 60% according to the observer had 5 or more ADHD symptoms of inattention and/or hyperactivity/impulsivity considering the 5 symptoms cut-off based on the recent version of the DSM (DSM-5; APA, 2013) criteria for ADHD in adults.

The 43 participants without ADHD were not part of the previous sample and they were selected according to the criteria of the National Institute of Statistics of Spain for the distribution of the population, based on their grade level and employment status in the age range of 17-24 years. We contacted these individuals by disseminating information about this research project in Universities, vocational training schools, and secondary schools and their participation was voluntary. They also had to meet the following criteria: 1) No history of significant problems of inattention or hyperactivity / impulsivity; 2) T scores below 65 on both the DSM-IV inattention and hyperactivity/impulsivity subscales of the CAARS observer and self-report versions; 3) Absence of neurological disorders, sensory or motor deficits (paralysis, deafness, blindness), autism or psychosis; and 4) an IQ equal to or greater than 70. Eight participants were removed for meeting 5 or more symptoms of inattention or hyperactivity/impulsivity according to the DSM-5 criteria. The group without ADHD had a mean age of 19.26 (SD=1.83) and a mean IQ of 103.02 (SD=11.57) (see Table 1).

The two groups, with ADHD and no ADHD, were matched for age, t (91)=- 1.56, p=.121 and sex, χ2 (1, N=93)=.208, p=.649, with 96.8% being males. The difference between the two groups in estimated IQ, using the vocabulary and block design tests from the Wechsler Adult Intelligence Scale (WAIS III; Wechsler, 1999), was not significant, t (91)=.42, p=.679.

Parents and participants reported drinking alcohol or consumption of tobacco, marijuana and other illegal drugs (e.g., cocaine, ecstasy, opiates, etc.) in the previous twelve months. This information was adopted as the criterion for splitting ADHD participants into two groups: a group with substance use (ADHD-SU) and a group without substance use or with low risk of substance use (ADHD-N/SU). We classified a participant in the former group when either he or his parents reported: 1) frequent consumption (2 drinks per day) or very frequent alcohol consumption (more than 2 drinks per day or more than 4 during the week); 2) any amount of smoking daily; 3) marijuana at least once a week; or 4) occasional use (at least once a month) of other illegal drugs, such as cocaine, heroin and opiates. A subject was classified in the latter group if both he and his parents reported that there was no SU or only very occasional consumption of alcohol or tobacco (e.g., at parties or other special occasions).

By dividing the ADHD group into two groups, one with SU and one without it, statistically significant differences in age were found (F(2,90)=4.36, p=.016); therefore, age was introduced in the analysis as a covariate.

The presence and severity of ADHD symptoms was evaluated with the long version of the Conners Adult ADHD Rating Scale-Observer version (CAARS; Conners, Erhardt, & Sparrow, 1999), which consists of 66 items divided into nine subscales. It includes the 18 ADHD symptoms from the DSM-IV, measured on a 4-point Likert-type scale and grouped in two subscales: DSM-IV-Inattention and DSM-IV-Hyperactivity/ impulsivity. The T-scores of these two subscales were used as dependent variables in this study. T-scores greater than or equal to 65 were considered clinically significant. DSM-IV-Inattention subscale difficulties include trouble concentrating, difficulty planning or completing tasks, forgetfulness, absent-mindedness, and being disorganized. The hyperactivity/impulsivity subscale includes items related to problems with working on the same task for long periods of time, feeling more restless than others seem to be, and fidgeting. This instrument has satisfactory reliability and validity. Internal reliability of the factor scales ranged from .86 to .92; test-retest reliabilities ranged from .88 to .91 (Erhardt, Epstein, Conners, Parker, & Sitarenios, 1999).

Questionnaire on Comorbidity-Weiss Symptom Record (WSR; Weiss, 2010). This clinical screening questionnaire is not psychometrically validated but is based on the DSM-IV-TR criteria (APA, 2000). Although it is not a diagnostic scale, it serves the useful purpose to identify adult mental disorders by collecting information about Axis I and Axis II mental disorders in any age group and from any informant. In this research was use the subscales used were Opositional Defiant Disorder (ODD), Conduct Disorder (CD), Anxiety, Depression, Sleep Disorder, and Antisocial Personality Disorder. The items have different response options; “0-Not at all”, “1-somewhat”, “2-pretty much” and “3-very much” (usually interpreted as “this is a problem that causes great difficulty”). A sum of the score on each item of the disorders mentioned above was used as dependent variable in the analysis dichotomized as “absence”(0, 1) or “presence” (2, 3). To evaluate clinical information about the pattern of SU, both the parents and young adults filled out the WSR subscale for Substance Use Disorder. This subscale is composed of 5 items about the consumption of alcohol, tobacco, marijuana and other street drugs, as well as the abuse of prescription drugs. In the current study, we did not use “abuse of prescription drugs” because none of the participants used these drugs.

The first aim of this study was to compare the SU of young adults with and without ADHD. Chi-square analysis showed statistically significant differences between the groups in the use of alcohol [χ2 (1, N=93)=5.91, p=.015], tobacco [χ2 (1, N=93)=10.22, p=.001] and marijuana [χ2 (1, N=93)=7.79, p=.005]. In all cases, SU percentages were higher in the ADHD group than in the no ADHD group. Differences in other illegal drugs, like cocaine or opiates, did not reach statistical significance [χ2 (1, N=93)=.250, p=.114] (see Table 2).

Table 3 shows that 54% (n=27) of ADHD adults consumed some kind of substance (ADHD-SU group) versus 46% (n=23) who did not (ADHD-N/SU group). Comorbid problem symptomatology in the two ADHD groups (with and without SU) was compared. MANCOVA results revealed statistically significant differences [Wilks’ Lambda (Λ)=.69, F (6, 42)=3.07, p=.014, η2p=.305]. ANCOVA results indicated statistically significant differences for ODD and CD. In both cases, the ADHD-SU group presented significantly higher comorbidity scores than the ADHD- N/SU group. However, ADHD-SU and ADHD- N/SU did not differ significantly on anxiety, depression, sleep disorder, or antisocial personality disorder (see Table 3).

Next, an analysis was performed of possible differences in ODD and CD manifestations in the two groups of subjects with ADHD. Specifically, there were statistically significant differences between ADHD adults with and without SU on some specific ODD symptoms, such as “Touchy or easily annoyed by others” [χ2 (1, N=50)=4.30, p=.030]; “Deliberately annoys people” [χ2 (1, N=50)=7.03, p=.008]; “Argues with adults” [χ2 (1, N=50)=4.72, p=.030] and “Loses temper” [χ2 (1, N=50)=4.78, p=.029]. There were no significant differences in the other ODD manifestations (see Figure 1).

Figure 1.

Percentage of presence of specific ODD symptoms in ADHD adults with and without substance use. ADHD-SU=ADHD with substance use; ADHD-N/SU=ADHD without substance use; ODD=Oppositional defiant disorder.

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There were also differences between the groups in some specific CD symptoms: “Stealing items of nontrivial value without confronting victim” [χ2 (1, N=50)=5.81, p=.016]; “Often lies to obtain goods or benefits or avoid obligations” [χ2 (1, N=50)=4.71, p=.030]; “Broken into a house, building, or car” [χ2 (1, N=50)=3.78, p=.05]; “Bullies, threatens, or intimidates others” [χ2 (1, N=50)=3.79, p=.05] (See Figure 2).

Figure 2.

Percentage of presence of specific CD symptoms in ADHD adults with and without substance use. ADHD-SU=ADHD with substance use; ADHD-N/SU=ADHD without substance use; CD=Conduct Disorder.

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Logistic regression analyses were conducted to examine the relationships among ADHD symptoms, externalizing problems (ODD and CD) and the risk of SU (see Table 4). The final models, where age, ADHD symptoms and externalizing problems were introduced, explained 21.2%, 31.5%, 44.3% and 49.6% of the variances in alcohol, tobacco, marijuana use and illegal drug use, respectively. These analyses revealed that neither ADHD symptoms nor conduct problems were associated with alcohol or tobacco use. The only variable that significantly distinguished the SU groups was conduct disorder, which was associated with both marijuana use (p=.038; OR=1.45; CI=1.02-2.07) and the use of other illegal drugs (p=.007; OR=1.95; CI=1.20-3.16). ADHD symptoms did not contribute significantly to any of the four final models; although inattention had a nearly significant association with marijuana use (p=.056; OR=1.09; CI=0.99-1.19).

The results from this study should be considered in the context of several unique study features and limitations. The group of young adults with ADHD was relatively small and had clinical remission. Therefore, it would be interesting to replicate the study with a larger sample. In addition, all of the participants in the sample with ADHD were diagnosed with the combined type from the DSM-IV, so generalization of the results should generally not extend beyond this subtype. Especially because both ADHD subtypes differs significantly in attention and in state and trait anxiety (González-Castro, Rodriguez, Cueli, García, & Alvarez-García, 2015). Moreover, comorbid problems were reported using a screening measure. These results should be replicated using instruments to establish a clinical/psychiatric diagnostic. In this study, the moderating role of socio-environmental risk variables was not examined either, such as parents’ educating styles or friends’ attitudes towards consumption and easy access to drugs, among others.

Overall, the relevance of our findings consists of their contribution to underscoring that young adults with an ADHD diagnosis have an increased risk of SU, specifically tobacco and marijuana, and therefore require preventive and early intervention efforts. Comorbid ADHD and SU produce significant and unique treatment challenges, including less likelihood of successful treatment completion, a longer time to SU recovery, and earlier SU relapse. Unfortunately, ADHD medication does not protect the individual from the risk of SU (Molina et al., 2013), and its efficacy in the treatment of co-occurring ADHD and SU is only modest (Cunill, Castells, Tobias, & Capellà, 2015), which suggests the need to identify alternative or adjunct approaches to substance abuse prevention and ADHD treatment.

Psychotherapeutic intervention in addition to pharmacological treatment can be beneficial. Cognitive behavioral therapy may be of particular interest, and neurofeeedback (given preliminary evidence suggesting its possible efficacy in adolescents with comorbid ADHD and SU (Thurstone, Riggs, Salomonsen-Sautel, & MikulichGilbertson, 2010).