Smoking is a lifelong addiction...

[Lord Magus]

Ran a quick check...

The Sign of Four opens with an alarming scene:

Sherlock Holmes took his bottle from the corner of the mantel-piece and his hypodermic syringe from its neat morocco case. With his long, white, nervous fingers he adjusted the delicate needle, and rolled back his left shirt-cuff. 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 arm-chair with a long sigh of satisfaction.


A little later in the story Holmes states,


"It is cocaine," he said, "a seven-per-cent solution. Would you care to try it?"

Dent was correct... I thought it was Heroine because I remembered the needle and the puncture marks, forgot the explanation though. I was mistaken, sorry.