Sherlock Holmes’s use of cocaine is among the most enduring aspects of Arthur Conan Doyle’s famous detective. First introduced in The Sign of Four (1890), Holmes’s voluntary injection of a seven-percent cocaine solution appears at first glance to be an eccentricity—a dramatic character quirk for literary effect. However, upon closer scrutiny, Holmes’s drug habit emerges as a complex element of his characterisation, reflecting the intellectual, emotional, and cultural tensions of his era. Far from incidental, the use of cocaine provides insight into Holmes’s psychological landscape, the role of addiction in Victorian society, and Conan Doyle’s nuanced vision of the costs of modern rationalism. The most direct and revealing reference to Holmes’s drug use comes at the beginning of The Sign of Four, when Watson enters their Baker Street sitting room and finds Holmes preparing an injection. When asked whether the drug is morphine or cocaine, Holmes replies, “It is cocaine… a seven-per-cent solution. Would you care to try it?” Holmes explains that he uses the drug during periods of inactivity, when his mind, lacking intellectual stimulation, becomes listless and restless. “My mind,” he tells Watson, “rebels at stagnation.” This phrase is of fundamental importance. Holmes views the drug not as an escape from reality but as a substitute for it—a temporary replacement for the rush of deductive exertion. It is a chemical analogue for the thrill of a new case. Watson’s reaction is notable both for its moral clarity and its professional sobriety. As a physician, Watson is deeply disturbed by the practice. He warns Holmes of the drug’s physiological consequences, referring to “the black reaction” that follows the euphoria, and of the long-term damage to both the brain and the nervous system. But more than professional concern, Watson’s response reflects personal affection. He is not merely a medical man witnessing pathological behaviour; he is a friend watching someone he respects and cares for engage in a harmful routine. This is not the only time that Holmes’s mental condition appears in the Canon in ways suggestive of substance use or its withdrawal. On several occasions, Holmes is described as sinking into long periods of torpor when no cases present themselves. In The Man with the Twisted Lip, Watson discovers him in an East End opium den—though it transpires that Holmes is in disguise, not partaking. These instances contribute to a picture of a man who functions at full capacity only when intellectually engaged and who is otherwise prone to severe ennui. The Canon implies, rather than states, that Holmes’s drug use occurs during such lulls, but the inference is clear: the drug becomes a remedy for an intolerable lack of mental stimulation. It is worth noting that Holmes’s use of cocaine was not an illegal act at the time of publication. Cocaine had been isolated in the mid-nineteenth century and by the 1880s was widely used in the medical profession, particularly as a local anesthetic. Sigmund Freud had, in 1884, published Über Coca, extolling its effects as a mood elevator and a stimulant. British pharmacological journals reported on its success in surgery, and it was found in tonics, lozenges, and even wine. In late Victorian London, one could walk into a chemist and purchase cocaine without prescription, either as a powder or in prepared solutions. It was not until the passage of the Dangerous Drugs Act of 1920 that the sale and use of cocaine became formally restricted under British law. Holmes, being of scientific mind and upper-middle-class status, would have had no difficulty obtaining cocaine. Residing at 221B Baker Street in Marylebone, a well-connected and central district, Holmes would have had access to a range of reputable chemists. Nearby Wigmore Street and Oxford Street housed several large pharmaceutical establishments known for compounding preparations to specification. Holmes’s preferred seven-percent solution—mentioned twice in the Canon—suggests he had access either to bespoke pharmacy service or made the preparation himself. Given his familiarity with laboratory equipment and chemical processes, it is plausible that Holmes purchased the base drug in powdered form and dissolved it himself in distilled water. His syringe, described as kept in a Moroccan leather case, was a relatively new invention but not uncommon by the 1880s. Intravenous delivery,