Background: Intravenous misuse and attention-deficit/hyperactivity disorder (ADHD) are common in patients under opioid maintenance treatment (OMT), who often misuse benzodiazepine (BZD). Objectives: To explore the rate of adult ADHD among patients under OMT in Italy and whether screening positive for adult ADHD is associated with OMT and BZD misuse and emergency room (ER) admission because of misuse. Methods: We recruited 1,649 patients from 27 addiction units (AUs) in Italy and collected data on the self-reported rate of OMT intravenous misuse (prevalence, repeated misuse, main reason, temporal pattern in relation to AU access, experience), concurrent intravenous and intranasal BZD misuse (prevalence, type of misused BZD), ADHD and ER admissions because of misuse complications. Results: Screening positive for adult ADHD was found in 11.2% patients (ADHD+), with a significant gender difference (women: 15.3%, men: 10.3%). OMT misuse was reported by 24.4 and 18.5% patients during lifetime and in the previous 6 months respectively. BZD misuse was reported by 20.0 and 8.6% patients for intravenous and intranasal route respectively. Misuse was significantly more common in ADHD+ (OMT 27.4–33.1%, BZD 14.5–31.5%) than ADHD– group (OMT 17.4–23.3%, BZD 7.9–18.3%). The multivariate logistic regression model showed positive screening for ADHD to be significantly associated with intravenous OMT misuse in the previous 6 months, and intravenous/intranasal BZD misuse, independently of age, gender and route of previous heroin administration. Conclusions: Screening positive for adult ADHD was associated with OMT and BZD misuse. AU physicians and medical personnel should focus on OMT patient’s features that are associated with a higher likelihood of misuse, in particular ADHD.