Literature and medicine are not usually seen as related disciplines, but scholars have already begun producing fruitful scholarship regarding historical and aesthetic interactions between them. This thesis adds to that scholarship by examining medicine and literature in nineteenth-century Britain. More specifically, Thomas De Quincey's Confessions of an English Opium-Eater and Robert Louis Stevenson's Strange Case of Dr. Jekyll and Mr. Hyde both use nineteenth-century medical case conventions to tell their stories. Furthermore, because both works deal with addiction, divided selves, and the power that physical substances can have on morality and character, these two works provide an excellent comparison coming 65 years apart. As such, they are a great point from which to begin looking more closely at how the interactions between medicine and literature evolved during the nineteenth-century in Britain. This thesis examines the role that "scientific" discourse has played in medicine and literature as interpretive disciplines, the rhetorical techniques and innovations surrounding the intersection of the two disciplines, and the authority that each discipline derived by implicitly borrowing ideological assumptions and textual forms from the other. Confessions is a wonderful example of a Romantic, autobiographical text that clearly uses the medical case study conventions; in fact, De Quincey was often cited in the years following the publication of Confessions as an authority on opium and its uses. By the time Jekyll and Hyde was published, however, a work like Confessions could no longer hold its own in medical debates. The professional institutions of medicine and literature had changed too much. Hence, by analyzing these two works side-by-side, I intend to illustrate different narrative approaches to similar issues at the beginning and end of the century. More importantly, I hope to use these texts in conjunction with specific medical case histories to discuss each text's reliance on interdisciplinary authority.