This paper analyses the determinants of social policy through a study of a crucial stage in the evolution of British narcotics legislation. Conditions in the First World War fundamentally altered the way in which narcotics were controlled in England and established a ‘hard-line’ reaction to drug use later reflected in the first Dangerous Drugs Act (1920) and the debates of the 1920s. Wartime needs formulated a pattern of governmental responsibility which, with control vested in the Home Office, still persists. The paper analyses the tendencies inherent in nineteenth-century poisons legislation, and argues that, despite Britain's reluctant adherence to the American-inspired system of international narcotic control, domestic narcotics legislation as considered prior to the outbreak of war was more liberal than the wartime regulation. Drug smuggling from England to the far east and fears, largely illusory, of a cocaine ‘epidemic’ in the army in 1916 brought more stringent regulation. Narcotic controls in Britain appeared set on a path similar to that of America's Harrison Act, which was being interpreted in an absolutist way. Only the report of the Rolleston Committee on Morphine and Heroin Addiction in 1926 marked a victory for the medical approach, but the influence of the events of 1916 lived on in other ways.