["Addiction, EarlyView. ", "\nAbstract\n\nBackground and aims\nOn 9 June 2022, Thailand changed its cannabis legislation, expanding access to individuals aged >18 years. Evidence suggests that such changes could increase the risk of cannabis use and related harms, and could influence patterns of use of other substances among adolescents. This study measured the potential impact of these legislative changes on adolescent substance‐related hospital admissions.\n\n\nDesign\nAn interrupted time‐series study to assess the impact of cannabis legalisation on substance‐related hospitalisations using anonymised admission records from the Thai Health Information Portal.\n\n\nSetting\nNationwide study in Thailand spanning from 1 October 2016 to 30 September 2023, with 9 June 2022 marking cannabis legalisation.\n\n\nParticipants/cases\nAdolescents aged 10–19 years in Thailand.\n\n\nMeasurements\nBased on International Classification of Diseases, 10th Revision diagnostic codes, hospitalisation records were classified as relevant or irrelevant to each of eight predefined substance categories: stimulants (excluding cocaine), alcohol, cannabis, sedatives (including opioids and narcotics), volatile solvents, nicotine, hallucinogens and cocaine.\n\n\nFindings\nThere were an additional 25.5 weekly cannabis‐related hospitalisations (95% prediction interval = 23.5–27.2) compared with counterfactual predictions, resulting in 1762 excess cases over the 68‐week post‐legalisation period. Statistically significant excess admissions were also observed for stimulants, nicotine, volatile solvents and hallucinogens, corresponding to additional admissions of 9.1, 1.4, 0.5 and 0.5 per week, respectively (95% prediction intervals = 0.3–15.4, 0.4–2.1, 0.3–0.7 and 0.2–0.7, respectively). No statistically significant changes were observed for alcohol‐, sedative‐ or cocaine‐related hospitalisations.\n\n\nConclusions\nCannabis legalisation in Thailand was followed by a marked rise in adolescent hospitalisations related to cannabis, alongside an increase in stimulant‐, nicotine‐, volatile solvent‐ and hallucinogen‐related hospitalizations. These findings highlight the potential unintended consequences of cannabis legalisation in Thailand and underscore the need for preventive strategies to reduce adolescent exposure and related harm.\n\n"]