Adherence to treatment is one of the most consistent factors associated with a favorable addiction treatment outcome. Little is known about factors associated with treatment adherence in individuals affected with comorbid attention-deficit/hyperactivity disorder and substance use disorders (SUD). This study aimed to explore whether treatment-associated factors, such as the prescribing physician's (sub)specialty and methylphenidate (MPH) dose, or patient-related factors, such as sex, age, SUD subtype, and psychiatric comorbidity, were associated with adherence to MPH treatment. Swedish national registers were used to identify adult individuals with prescriptions of MPH and medications specifically used in the treatment of SUD or a diagnosis of SUD and/or coexisting psychiatric diagnoses. Primary outcome measure was days in active MPH treatment in stratified dose groups (≤36 mg, ≥37 mg to ≤54 mg, ≥55 mg to ≤72 mg, ≥73 mg to ≤90 mg, ≥91 mg to ≤108 mg, and ≥109 mg). Lower MPH doses (ie, ≤36 mg day 100) were associated with treatment discontinuation between days 101 and 830 (HR≤36 mg, 1.67; HR37-54mg, 1.37; HR55-72mg, 1.36; HR73-90mg, 1.19; HR≥108mg, 1.09). The results showed a linear trend (P < 0.0001) toward decreased risk of treatment discontinuation along with increase of MPH doses. In conclusion, this study shows that higher MPH doses were associated with long-term treatment adherence in individuals with attention-deficit/hyperactivity disorder and SUD.