Sherlock Holmes was a crackhead.
From the first chapter of The Rule of Four:
Sherlock Holmes took his bottle from the corner of the mantelpiece, and his hypodermic syringe fom its neat morocco case. With his long, white, nervous fingers he adjusted the delicate needle and rolled back his left shirtcuff. For some little time his eyes rested thoughtfully upon the sinewy forearm and wrist, all dotted and scarred with innumerable puncture-marks. Finally he thrust the sharp point home, pressed down the tiny piston, and sank back into the velvet-lined armchair with a long sigh of satisfaction.
Three times a day for many months I had witnessed this performance…"Which is it to-day," I asked, "morphine or cocaine?"
He raised his eyes languidly from the old black-letter volume he had opened.
"It is cocaine," he said "a seven-per-cent solution. Would you care to try it?"