Psychiatrists and other mental health professionals used to assume that the characteristics of autism spectrum disorder (ASD), such as poor social and communication skills and a tendency to avoid social situations and abide by rules, would be protective against substance use disorder (SUD), and that ASD cases co-morbid with SUD would be uncommon. However, these assumptions have recently been challenged in part due to the changes in defining ASD, resulting in more variability in its phenotypes. Recent cases of ASD do not match the typical characteristics; thus, it is necessary to re-examine ASD/SUD co-morbidity. For this purpose, we reviewed available literature on the prevalence of ASD/SUD co-morbidity, psycho-social aspects of substance use by individuals with ASD, and assessment and treatment issues. We reviewed 11 research papers on children and adolescents with ASD and 13 on adults with ASD. Our literature review suggested that the prevalence of ASD/SUD co-morbidity may be higher than previously thought, and that ASD as a risk factor for substance use may become more problematic as individuals make a transition into adulthood. One explanation for this heightened prevalence, as described in the reviewed literature on the psycho-social aspects of substance use by individuals with ASD, is high social demands exceeding the individuals' innate capacities. Such social demands may induce mental distress to which these individuals would be more vulnerable than the general population, and for whom substance use would possibly function as self-medication for negative emotions and improved social functioning. Thus, in terms of SUD assessment and treatment for individuals with ASD, treating professionals should pay close attention to what role substance use plays in the lives of individuals with ASD. Previous studies found that nonabstinent reduction could be a reasonable option as a treatment goal for alcohol use disorder (AUD); however, we believe that this should be decided according to the AUD severity of each individual patient. In terms of therapeutic interventions, the characteristics of ASD should be taken into consideration. Less emphasis should be placed on the importance of group modality in favor of a more individual-oriented treatment modality, and utilization of more supportive and directive rather than insight-oriented approaches. Furthermore, social and vocational support to help structure the everyday lives of individuals with ASD would be beneficial in addition to SUD treatment. Because many of the reviewed research papers were published abroad, mostly in European and North-American countries, the differences in national policies and public attitudes on substance use necessitate mental health professionals in Japan conducting further research to apply these findings to Japanese ASD and SUD patients.
 Authors' abstract