Cocaine

"[...], while Holmes, who loathed every form of society with his whole Bohemian soul, remained in our lodgings in Baker Street, buried among his old books, and alternating from week to week between cocaine and ambition, the drowsiness of the drug, and the fierce energy of his own keen nature."
(Aventure I, A SCANDAL IN BOHEMIA pg. 3)

While the stimulant and hunger-suppressant properties of coca had been known for many centuries, cocaine was first isolated in 1855 by the German chemist Friedrich Gaedcke. Cocaine is presented as a vice in Arthur Conan Doyle's story, and is often used by Sherlock Holmes to offset boredom and as a way to stimulate his brain.