Who and where was Sherlock before we met him in Saint Bart's morgue?
But this a modern Sherlock cannot be a drug addict. Yet, it's so much a part of the Canonical Holmes, it must be accounted for. And so it merges in snippets of dialogue and subtext of actions:
Sherlock Holmes is a recovering drug addict.
But our Sherlock is also a drug addict, though recovering, apparently. We have one reference and explanation presented early in S1E1, when Watson walks in on Sherlock in what looks like mid-shoot-up, only to be shown nicotine patches he's just placed on his arm, for "brain work." The viewer then easily dismisses any idea that the clear-eyed, clean-jawed Holmes is shooting, snorting or swallowing anything illegal or more addictive than nicotine into his body.
Later in this episode, Lestraude stages a fake "drugs-bust" of Sherlock's just moved-into flat and John, quickly loyal and protective despite his earlier protestations to Mycroft, jumps to his defense, exclaiming, "This guy? A junkie? Have you met him?" But Sherlock advises John to "shut up" and gives him a rather significant quelling look.
We don't have time to make anything of it as the Sherlock solves the "Rachel" clue and the killer cabbie demands attention. Sherlock is soon off to be talked into suicide, leaving John desperately searching for him. On reflection, John had only known Sherlock for about a day, Lestrade had known him for 5 years. John defending Sherlock to Lestrade as someone who would never use drugs, was what Sherlock wanted to stop.
Later, inside the empty school, the kamikaze cabbie says something in those last moments, pushing Sherlock to actually put the pill he is holding into his mouth:
"What's the point of being clever if you can't prove it ... still the addict ... but this, this is what you're really addicted to ... you'll do anything, anything at all, to stop being bored..."
Still the addict? Still? When was he before and how would the cabbie know? Moriarty told him. All the words to manipulate Sherlock come from Moriarty. Just as he later tells Irene Adler how to "play the Holmes boys." And, just like Mycroft, Moriarty also knows about people like Sherlock, those who can threaten him.
Why was Sherlock looking for a flat? Where was he living before? And why would he be looking for a flatmate? While Sherlock was identifying Irene Adler's body in the morgue with Mycroft, what were John and Mrs. Hudson searching the flat for? Why was it so significant that Sherlock took a cigarette from Mycroft?
Why did John have to cancel his date in the middle to stay home with Sherlock? Surely this level of concern wasn't a reaction to the fact that
he might fall off the smoking wagon. And why, when Sherlock came home and saw that the flat had been searched, was he not surprised? It had been searched before, as he indicates with his comment: "I hope you didn't mess up my sock index, this time."
Our 21st century Sherlock Holmes is a drug addict. Which is why when he is idle, bored, he is sometimes frantic to find something to engage his mind, to keep himself clean. As his 19th century alter-ego said:
"Give me problems, give me work, give me the most abstruse cryptogram, or the most intricate analysis, and I am in my own proper atmosphere, I can dispense then with artificial stimulants
(Holmes in "The Sign of Four")
See also: SHERLOCK BACKSTORY 2: the Privateer