Clinical implications of ADHD, ASD, and their co-occurrence in early adulthood—the prospective ABIS-study

BACKGROUND: Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are childhood-onset disorders associated with functional and psychosocial impairments that may persist into adulthood, leading to serious personal and societal costs. OBJECTIVE: This study aimed to examine the socio-economic difficulties, physical and mental comorbidities, and psycho-social vulnerabilities associated with ADHD, ASD, and their co-occurrence among young adults. METHODS: 16 365 families with children born 1997–1999, were involved in the prospective population-based ABIS study (All Babies in Southeast Sweden). A total of 6 233 ABIS young adults answered the questionnaire at the 17–19-year follow-up and were included in this case–control study. Diagnoses of ADHD and ASD from birth up to 17 years of age were obtained from the Swedish National Diagnosis Register. N=182 individuals received a single diagnosis of ADHD, n=78 of ASD, and n=51 received both diagnoses and were considered the co-occurrence group. Multiple multinomial logistic regression analyses were performed. RESULTS: In the univariate analyses all three conditions were significantly associated with concentration difficulties, worse health quality, lower socio-economic status, lower faith in the future, less control over life, and lower social support. In the adjusted analyses, individuals with ADHD were almost three-times more likely to have less money compared with their friends (aOR 2.86; p < .001), experienced worse sleep quality (aOR 1.50; p = .043) and concentration difficulties (aOR 1.96; p < .001). ASD group were two-fold more likely to experience concentration difficulties (aOR 2.35; p = .002) and tended not to have faith in the future (aOR .63; p = .055), however, showed lesser risk-taking bahaviours (aOR .40; p < .001). Finally, the co-occurrence was significantly associated with unemployment (aOR 2.64; p = .007) and tended to have a higher risk of autoimmune disorders (aOR 2.41; p = .051), however, showed a 51% lower risk of stomach pain (aOR .49; p = .030). CONCLUSIONS: All these conditions significantly deteriorated several areas of life. ADHD/ASD co-occurrence is a heavy burden for health associated with several psychosocial vulnerabilities, that shared a similar morbidity pattern with ADHD although showed less risk cognitive and behavioral profile, similar to the ASD group. Long-term follow-up and support for individuals with these conditions over the life course are crucial. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12888-023-05298-3.