Did Sir Arthur Conan Doyle do drugs?

Holmes stated in at least one place (Sign of the Four) that his mind was like an engine; give it a problem and it could run all day and night, but with no load, his mind foundered. As such, he was prone to take opium and cocaine, but only during times of laxity, between cases.

IMPORTANT NOTE: Using narcotics like cocaine and opium was LEGAL in Victorian England.