The current study used strict criteria for abstinence, which required at least 30 days of abstinence, to avoid confounding symptoms of intoxication or withdrawal. On the other hand, other studies required 4 days of abstinence prior to the interview . Since the diagnosis of adult ADHD depends on the presence of significant clinical symptoms in childhood, the researchers used KSAD-PL to investigate this part of patients’ lives and to ensure that the current symptoms of ADHD are not secondary to intake of drugs. It is noteworthy to note that none of the potential cases of ADHD received medication to treat ADHD during their childhood period. The current study showed that the number of potential ADHD cases according to CAAR-S:L nearly doubled that of the controls, consistent with previous studies [14, 15].
Potential adult ADHD among the present sample of patients seeking treatment from SUD was 17.65% according to the CAARS-S:L ADHD index and past history of childhood ADHD was 15.69% according to K-SAD-S PL DSM-IV. The current result is nearly similar to the results of the meta-analysis by Van Emmerik-van Oortmerssen et al., which revealed an overall rate of 22% . In this meta-analysis, studies focused on treatment-seeking patients showed ADHD prevalence of 23.3%, while community-based studies reported ADHD prevalence of 44.3% and 15.5%, among adolescent and adult populations, respectively . Interestingly, no significant association was observed between clinical variables such as male gender, age, or the study setting and ADHD prevalence [6, 16].
The comorbidity with ADHD is influenced as well by the type of instruments used, for instance, a study using the Diagnostic Interview for Children and Adolescents (DICA) or the Schedule for Affective Disorders and Schizophrenia—Lifetime Version (SADS-L) for the diagnosis of ADHD showed significantly higher comorbidity rates than studies using the Diagnostic Interview Schedule for Children (DISC), Diagnostic Interview Schedule for DSM-IV (DIS), or other assessment instruments .
As much as a diagnostic instrument used affected the prevalence of co-morbidity, the type of substance abuse played a significant role. Lower prevalence of ADHD was associated with cocaine than other substances. The prevalence among methadone maintenance patients was 24.9% . On the other hand, benzodiazepines addicts had a higher rate of probable ADHD reaching 31.7% of screened subjects and more associated with polysubstance abuse .
ADHD was over-represented among SUD populations. General population surveys indicate an average prevalence of 3–4% of adult ADHD [18,19,20], with a pooled estimated prevalence of 2.5% , whereas among treatment-seeking adult SUD patients, the prevalence of adult ADHD is substantially higher, ranging from 10 to 46% [6, 22, 23]. Possible explanations for this variability include differences in diagnostic criteria, primary drug of abuse, country-specific factors (treatment offer, service structure), treatment setting (e.g., inpatient versus outpatient treatment), clinical biases, and demographic factors.
In the current study, using CAARS as a diagnostic tool in SUD patients is consistent with a previous study by Dakwar et al. . In that study, every instrument tested demonstrated adequate sensitivity, specificity, and positive and negative predictive values, with the CAARS outperforming the rest overall, particularly when ADHD NOS (not otherwise specified) was labeled as not ADHD and exhibiting the greatest degree of agreement with the CAADID (Conners’ adult ADHD Diagnostic Interview for DSM-IV). Of the 3 instruments, (Wender Utah Rating Scale (WURS), CAARS, and the Adult ADHD Self-Report Scale-Version 1.1 (ASRS-V1.1)), CAARS adheres to DSM-IV criteria in the most comprehensive manner, thus explaining its superior agreement with the CAADID as the gold standard of the study. The ASRS-V1.1, also predicated on the DSM-IV, is much shorter at only 6 items, while the WURS draws on the Utah conceptualization of ADHD .
Previous studies that used K-SAD-S in diagnosing adult ADHD in SUD patients showed wide variations (8–44.3%), which could be explained by multiple factors especially the short period of abstinence before the interview that could lead to overestimation. Unfortunately, all studies using K-SAD-S were performed for adolescents rather than adult populations .
In the current study, patients coming from a high social class were predominant (54.9%). Also, the university graduates or students (60.78%) were highly represented. Other studies reported higher social classes in patients with SUDs with adult ADHD compared to those having SUDs only . Egyptian reports suggest more prevalence of SUDs among urban areas and less educated populations . However in the sample of this study, illiterate patients were excluded, while the overrepresented university students may seek treatment for substance use more commonly due to high awareness.
The previous studies differed in their ways to explain the relationship between the type of substance abuse and ADHD. Some clinicians have suggested that individuals with ADHD may preferentially use cocaine to “self-medicate” their underlying psychiatric disorders . Others attribute this association to increased impulsivity or feelings of social incompetence.
However, the higher rates of current marijuana use among cocaine abusers with adult ADHD suggest that other drugs, and not simply cocaine, are used by adults with ADHD. These findings are consistent with Biederman et al. who found that marijuana dependence, and not cocaine dependence, was the most common substance use disorder among adults seeking treatment for their ADHD symptoms . Marijuana may continue to help individuals with ADHD to “feel calm” despite its ability to produce other negative social and occupational consequences . Other researchers have found ADHD rates to be elevated in alcoholics as well as opiate abusers. A previous study reported that the alcohol use disorder outpatient adult ADHD prevalence rates ranged from 4 to 14% and the drug use disorder outpatient adult ADHD prevalence rates ranged from 10 to 33% .
Levin and colleagues suggested it may be that individuals with adult ADHD do not initially choose a specific class of substances, e.g., stimulants, but rather adult ADHD may be a significant contributing factor to substance abuse in general . However, the potential additive contribution of comorbid ADHD to drug-specific dependence in SUD populations is largely unknown. Drug dependence complexity and chronicity are increased in SUD patients with ADHD, particularly for alcohol, amphetamine, and opiates rather than heroin, methadone, and benzodiazepines .
The present study is consistent with multiple studies showing that individuals with ADHD diagnosis have an earlier onset of substance abuse than those without ADHD diagnosis, a greater likelihood of having continuous problems if they develop substance dependence, a reduced likelihood of going into remission, and a tendency to take longer to reach remission . The literature investigating the association between ADHD subtypes and SUDs is mixed. Some studies do not report a significant relationship , while others have suggested that hyperactive/impulsive symptoms are more associated with the risk for SUDs than inattentive symptoms . Furthermore, a study in adults with ADHD reported that the combined subtype had a higher incidence of lifetime SUDs than the inattentive subtype, suggesting the greater contribution of hyperactive/impulsive symptoms . Other studies—in contrast—reported that inattention has been associated with early illicit drug use, frequency and recency of alcohol and marijuana use, heavier cigarette use , tobacco and marijuana use, and nicotine dependence .
Using a meta-regression analysis of ASI values revealed that the most important factors affecting the ADHD index as a dependent variable were drug problems and legal status, while the most important factor that affected ADHD diagnosis by K-SAD-S as a dependent variable was the drug problems. Putting into consideration the wide range of problems included in the term “drug problems”, the result of regression analysis is considered consistent with other previously mentioned studies.