Addiction, or Substance Use Disorder (SUD), among adults with Autism Spectrum Disorders (ASD) has never been studied before, contrary to SUD in ADHD. However, we know from clinical practice that comorbid SUD in ASD can present with a considerable impairment. Distinguishing between ASD and ADHD can be challenging, but becomes increasingly so in case of comorbid SUD.

The main objective of this study was to gain insight into the aspects of comorbid SUD in ASD on three levels: a phenotypical, an endophenotypical, and a genetic level. A second objective was to determine how ASD differs from ADHD in adult patients on these three levels, with and without comorbid SUD.

We studied 130 patients with ASD (n = 75) or ADHD (n = 55) with or without SUD. We looked at risk factors for developing SUD, prevalence, and outcome. Furthermore, we studied personality profiles and autism spectrum profiles (AQ). We assembled neuropsychological and genetic data.

Comorbidity of SUD with ASD is less common than in ADHD, but the consequences are just as severe. Comorbid SUD is not always associated with a negative outcome, at least in the perception of the individual. Assessment of adults with ASD and SUD can be misleading when the neuropsychological functioning is not also taken into consideration. Finally, in the absence of SUD, ASD and ADHD in adults share many features on the (endo-)phenotypical level, but may be distinguishable at a genetic level. In the presence of SUD, differences between ASD and ADHD are more pronounced.