Attention deficit/hyperactivity disorder (ADHD) is an important contributing factor in the pathophysiology of substance use disorders (SUD), and ADHD occurs more often in populations with SUD compared to the general population. This high prevalence rate and comorbidity may relate to a shared neurobiological vulnerability, including a deregulation of the brain's dopamine system. This comorbidity directly impacts the treatment of ADHD: treatment with methylphenidate is less effective in patients with ADHD and SUD compared to ADHD patients without SUD.

We investigate the underlying neurobiological background of reduced treatment effectiveness for adult ADHD patients with comorbid SUD.

We observed lower available dopamine transporters, as well as a reduced binding of methylphenidate to these transporters, and more neurocognitive dysfunction in adults ADHD patients with SUD compared to ADHD patients without SUD.

Comorbid ADHD and SUD has a high prevalence rate and reduces ADHD treatment effectiveness, which makes it necessary to screen for the presence of ADHD in patients seeking SUD treatment. Neurobiological and neurocognitive differences are present between ADHD patients with and without SUD, which together may partially explain the reduced effectiveness of methylphenidate in adult ADHD patients with SUD.

The author has not supplied his declaration of competing interest.