The study aimed to identify similarities and differences among ketamine, methadone users, and non-drug-using controls, on impulsivity, antisocial personality, and related cognitive abilities. A case–control observational design was used to compare the impulsivity and cognitive function of ketamine users (n = 51), methadone users (n=59), and controls (n=60). Antisocial personality traits and emotion states were also measured. One-way ANOVAs and planned post hoc pair-wise tests were used to analyze the data. Compared to non-drug-using controls, ketamine and methadone users had elevated scores on BIS and Pd scale of the MMPI, poorer performance on 2-back task, Stop-signal task, and Stroop test. Ketamine users performed the worst in the 2-back accuracy and Stop miss rate compared to methadone users and controls. There were no significant differences between the groups on the Iowa Gambling Task. Ketamine users did not show deficits in decision-making but exhibited strong impulsivity, antisocial personality, and poor response inhibition and working memory at levels similar to methadone users. These deficits may reflect vulnerability to addiction. This suggests that future treatment programs for ketamine users could address drug users’ impulsive cognition and psychopathic deviance.