The South Asian region, home to approximately 22% of the global population, faces significant challenges related to substance use due to its proximity to major illicit drug-producing regions, the Golden Triangle and the Golden Crescent. This chapter explores the intersection of substance use and forensic psychiatry in South Asia across four key domains: (1) the legal framework surrounding the sale, production, and consumption of illicit substances; (2) the diverse policy responses to substance use in the region; (3) substance use in prison settings, including the implications for rehabilitation and recidivism; and (4) forensic assessments related to offenses committed under intoxication, including capacity evaluations and criminal responsibility. Given the high burden of substance-related offenses and the significant treatment gap, there is a pressing need for enhanced regional collaboration, such as under the SAARC Convention on Narcotic Drugs and Psychotropic Substances, to integrate psychiatric expertise into legal and policy frameworks. Strengthening forensic evaluations, improving access to addiction treatment within the criminal justice system, and fostering cross-border cooperation can help address the complex interplay of substance use, crime, and mental health in the region.