Objective: The purpose of this study was to investigate changes in narcissistic personality traits and empathy deficits in adults with attention-deficit hyperactivity disorder (ADHD) after pharmacotherapy, focusing on psychostimulant treatment. Understanding these comorbidities is crucial for optimizing treatment and improving therapeutic outcomes. Methods: This cross-sectional cohort study consisted of 75 randomly selected ADHD patients. The patients were started on methylphenidate or atomoxetine as per the indications for treatment. A Sociodemographic Data Form, Adult ADHD Self-Report Scale (ASRS), Pathological Narcissism Inventory (PNI), and Empathy Quotient (EQ) were applied. Based on the study design, Adult ASRS, PNI and EQ were re-administered to the patients at the end of 3 months of treatment. The levels of pathological narcissism and empathy in patients were assessed before and after treatment. Results: The median and minimum–maximum in PNI grandiosity score decreased significantly from 67.00 (30-108) before treatment to 60.00 (24-102) after treatment (P < .001). The mean ± Standard Deviation (SD) PNI Vulnerability score decreased statistically significantly from 63.83 ± 26.02 before treatment to 60.06 ± 24.64 after treatment (P=.003). The median and minimum–maximum in PNI total score was 134.50 (64-208) pre-treatment, which showed a statistically significant decrease after treatment (P < .001). The mean ± SD EQ score decreased statistically significantly from 33.86 ± 7.03 before treatment to 36.53 ± 7.55 after treatment (P=.001). Conclusion: Psychostimulant treatment in adult ADHD patients can ameliorate narcissistic traits and enhance empathy, thereby improving social functioning and interpersonal relationships. These findings suggest that psychostimulants may positively modulate neural circuits involved in narcissism and empathy, potentially leading to better treatment outcomes.