Holmes occasionally uses addictive drugs, especially when lacking stimulating cases. He believes the use of cocaine stimulates his brain when it is not in use. He is a habitual user of cocaine, which he injects in a seven-per-cent solution using a special syringe that he keeps in a leather case. Holmes is also an occasional user of morphine but expressed strong disapproval on visiting an opium den. These drugs were legal in late 19th-century England. Both Watson and Holmes are serial tobacco users, including cigarettes, cigars, and pipes. Holmes is expert at identifying tobacco-ash residues, having penned a monograph on the subject.
Dr. Watson strongly disapproves of his friend's cocaine habit, describing it as the detective's "only vice" and expressing concern over its possible effect on Holmes's mental health and superior intellect. In later stories, Watson claims to have "weaned" Holmes off drugs. Even so, according to his doctor friend, Holmes remains an addict whose habit is "not dead, but merely sleeping".
