Throughout the nineteenth century, opium occupied a position of great significance within the British Empire, comprising by the 1870s as much as 17% of the total revenues of the British India Government. Opium also made up the greater part of British exports to China, a legacy of early-century market exploitation and highly favourable commercial treaties signed following two opium wars fought by the two countries. Between the emergence of an organised anti-opium movement in 1875 and the close of the final international Geneva Opium Conference of 1931, British opium policy experienced a complete transformation. The development of British responses to the issue of opium offers a case study in the cultural history of international relations, while also offering insights into developments in the political scene in Britain. A critical issue at the heart of the transition from elite to mass politics in Britain at the crux of the emergence of a new socio-political landscape after the passage of the 1867 Reform Act, the increasing importance of public opinion and popular politics, the course of debate over opium characterised shifts in the British domestic political scene, highlighting the defining transitions in political action and social activism of the period. Opium was also a central focal point in the transformation of the global geopolitical environment at the turn of the twentieth century, with the emergence of hostile rival powers seeking to challenge British commercial and geopolitical pre-eminence, particularly in the form of the United States and Japan in Asia, with a radically different and reforming American approach to imperial policy in the region. This thesis examines these transitions, exploring the different phases of opium policy, and identifying the driving forces, causational factors, and continuities that defined these processes of reform, comprising a re-reading of the history of opium reform as a critical juncture within the cultural sphere of international relations.