Background: One of the public health concerns is connection between diagnosed attention-deficit/hyperactivity disorder (ADHD) and substance use disorders (SUD). According to several studies, having ADHD predisposes one to using both legal and illegal substances, especially those associated with SUD. Aim: This study was set out to determine how common adult ADHD was between people with substance use disorders. A comparison of the onset, severity, and the substance of usage in patients with adult ADHD and those without might also be intriguing. Patients and Method: This was a cross-sectional comparative study that included 200 inpatients with SUD picked randomly from the Addiction Treatment Center at Port Said Psychiatric Hospital, over 6 months from December 2021 to May 2022. Results: We found 57 (28.5%) of the participants have ADHD, while according to the Wender Utah Rating Scale (WURS) 119 (59.5%) had ADHD in their childhood. In ADHD participants' alcohol, tramadol, opioids, and stimulants were significantly higher than non-ADHD participants. Addiction Severity Index (ASI) analysis was statistically significant severity in all aspects of addiction severity index in patients with ADHD except psychiatric section. Conclusion: The adult ADHD strongly influences the development of SUD. Adult ADHD patients had higher rates of relapse, as evidenced by more frequent hospitalizations for substance abuse problems, earlier onset of substance use, and shorter abstinence intervals. Therefore, effective care for SUD should be included in screening for ADHD symptoms and including them in treatment regimens.