A/N: Stylistically, this is really different to the previous two parts. I hope you all like it :)
It's 3:30am. Sherlock quietly pushes open the door to his bedroom and emerges with a pile of dirty laundry, a thermos flask and a plate. He leaves the plate and the flask on the side in the kitchen, deposits his laundry in the washing machine and puts the kettle on.
In the room upstairs, John Watson listens carefully for any noise from below. He's been home for four days and so far, Sherlock has been successful in avoiding him. But if there's one thing John has learnt during his time with Sherlock, it's how to observe; how to deduce. Maybe if this incident had occurred earlier in their relationship, John would've given up and just waited for Sherlock to come out of his own accord instead of confronting him. But not now. Now, John is playing the game too. So far, he's certain of one thing - Sherlock has managed to avoid him thus far by only leaving his room at night, when John is asleep. Tonight, with the aid of caffeine, John plans to beat Sherlock at his own game.
Downstairs, a cupboard door slams. John jolts awake from his half-asleep state and smiles.
One floor below, Sherlock scowls at the state of the kitchen cupboards. So far the only edibles he's found are milk, stale crackers, a jar of pickled onions, some bread and a packet of Nik Naks. On the positive side, this means John is probably going shopping later on today, and although Sherlock suspects John will try to lure him (out of his room by hiding whatever food he buys, he knows finding it will be easy. John will hide it in nice, conventional places - the back of a cupboard, behind the TV, under the sofa. Sherlock smiles as the idea of turning the whole thing into a game crosses his mind, but the smile fades quickly when he hears the front door open and watches in surprise as John walks into the kitchen.
"You need to go shopping," Sherlock says, casually breaking the silence. "There's no food. When you do, remember to buy me some nicotine patches, I've almost run out."
John stares at the other man. Sherlock's uncanny casualness makes him almost forget his planned speech - this wasn't how this was meant to go.
"I'm not getting you anything until we talk," John says because it's the only thing he can think to say. Suddenly all the actions and words he's been planning seem stupid and foolish. Normally he doesn't mind confrontation, but there's something intimidating about Sherlock's presence that makes John think twice about seeing this through. But, no, he has to - this is important. "Well?" he presses, because he really doesn't know how to start this conversation.
Sherlock sighs heavily. A small part of him had been hoping that John would just drop the issue, but he knew that was unrealistic. Perhaps it would be best to just get it over with. "Cocaine, mostly," he says, just as John is about to ask him again. "Sometimes morphine. I might branch out on the odd occasion if I'm feeling adventurous."
"Cocaine?" John repeats, eyebrows raised.
Sherlock nods. "I find it a most transcendently stimulating substance."
"Yes. From Barts." Sherlock pauses. "Why are you so surprised?"
John ignores the question, because he's not really sure whether he's more surprised or unsurprised. "And the other day? What was that?" he asks instead, despite the fact that quite a large part of him really doesn't want to know.
"Cocaine," Sherlock says candidly. "I've still got some left, if you'd like to try," he adds, smirking slightly.
John lets out an exasperated sigh and turns away in frustration. As if he thought he could actually have a mature conversation about something with Sherlock, of all people.
"I see I've disappointed you again," Sherlock says. "You really should stop holding me up to such high standards."
"I'm not holding you up to high standards. I'm holding you up to your own standards," John says quietly, and turns to look at Sherlock. "If you care about your work so much, if all you care about is your brain, then why are you going to such lengths to destroy yours?"
"Oh, don't be stupid, Sherlock! Not even you can be unaware of the side effects. There's increased risk of infarction, stroke, paranoia, anxiety..." John lists, trying to think of as many cocaine-related health risks as possible.
Sherlock sighs. "Yes, I know."
John looks at him, and waits for a counter argument, but none comes. "Then... why?"
"Do you have any idea what it's like, being me?" Sherlock asks, but doesn't give John a chance to answer. "Of course you don't. So don't hold me up to the same standards as everyone else. I get bored, John, and when I get bored, my brain rots. Give me problems, give me work and I am fine, but faced with the dull routine of existence, I start to go mad and this is the only thing that stops it." He pauses for a moment. "It's a measured risk."
John blinks, dumbfounded. "You're doing this for fun? How... how is that a measured risk?"
Sherlock rolls his eyes. Trust this to be the only issue on which John is less open-minded and tolerant than normal. "It's a measured risk because I'm not like everyone else. You of all people should know that."
John scoffs. "You're not physically different, Sherlock. Taking drugs is still going to do you serious damage."
"Oh, what does that matter?" Sherlock exclaims. The conversation is one he's had many times and he's tired of having to explain himself. "Being bored does me serious damage."
John stares in disbelief at the other man. "You can't compare the side effects of an illegal drug to boredom"
"Because..." John trails off, desperately trying to search for a reason.
In the kitchen, the kettle begins to boil. A perfect diversion. "Why do you care so much?" Sherlock asks, and scans the kitchen to see if there are any clean mugs.
"Because I care about you, Sherlock! God knows why, but I do," John admits and adds in a quieter voice, "I've already had to watch my sister destroy her life. I am not watching you destroy yours."
"I'm not destroying it," Sherlock says brusquely as the kettle boils. "Tea?"
"You're taking illegal drugs! If that's not destroying your li-"
"You take sugar with your tea, don't you?" Sherlock asks, locating teabags and two mugs. John watches him, confused. "Two sugars. And not too much milk, am I right?"
"Sherlock, I don't... Why are you making me tea?"
Sherlock beings rifling through various containers in order to find the sugar. "Why shouldn't I? I thought you liked tea," he says and manages to locate the sugar. "Aha!"
"Now's not the time," John says, and wonders if he's managed to dream the entire conversation - Sherlock never makes tea. Ever. "We're having a conversation. It's... it's important."
"We were talking," Sherlock says. He finishes stirring his tea, picks it up, and walks towards his room. "I'm going to bed now," he calls, "Enjoy the tea."
John just stares.
Sherlock doesn't sleep that night. He nurses his tea long after he's finished drinking it. The feeling of unease and guilt for walking out on John, and the idea that John wouldn't tolerate his habits, stay in his mind and keep him awake. Sherlock knows he's not an addict in the conventional sense, but he also knows that he needs the drugs for when life gets quiet and boring; for when waking to This Morning and Jeremy Kyle and Cash in the Attic is too much; for when he starts to get that urge to do stupid and dangerous things 'just because.' While those times are (relatively) rare, they do occur, and if John didn't want to watch him 'destroy his life', did that mean he'd leave? Sherlock can't shake the idea and can't shake the fact that it scares him a little. He's never before realised just how much he's come to rely on John and just how much he needs him around. Pacing around his room, Sherlock comes to a conclusion: he needs to do something to convince John to stick around and tries desperately to think of something appropriate.
It's morning when he reaches a decision. Sherlock hesitates for a few minutes before executing his task, but he has to (however reluctantly) admit that John is the most important thing in his life. Immediately, he finds his coat and decides to go out before he can change his mind.
John doesn't sleep much either. He spends most of the night thinking about drugs and Sherlock, about what was said last night, and, in particular, about what Sherlock had said. He did have a point - boredom was bad for Sherlock (and the walls and anyone and everything in his general vicinity) but... John's mind was still uneasy when he thought about Sherlock taking drugs. He didn't seem to be addicted, but he could just be cleverly hiding it, and he could still become addicted in the future. But Sherlock was so stubborn. John couldn't offer him an ultimatum or tell him to quit because, more likely than not, Sherlock would just hide it and Sherlock is very good at hiding things. He finds himself drawing to the conclusion that maybe he should just tolerate it; he is in an ideal position to make sure that Sherlock's substance issues don't get any worse. John doesn't particularly like it, but it's something he can live with.
John goes downstairs later than he normally does. The flat seems empty and on the kitchen table, he finds a small bottle, half-filled with a clear solution. It's clearly been left there by Sherlock and John can't help but smile at the magnitude of the gesture.
Sherlock only returns when he is sure that John will be awake. He comes back to Baker Street at 10AM, and John sat in his chair, channel flicking. His cocaine is on the kitchen table, where he left it.
"I haven't been shopping yet," John says and turns the TV off. "We'll have to go out for lunch."
"Oh," says Sherlock, eyeing the bottle with surprise. "Well, I did buy some things from the shop," he adds, and awkwardly lifts up a plastic bag.
John stares at him in surprise. "You went shopping?"
"I didn't know what else to do," Sherlock admits. "It's only tea and nicotine patches. Oh, and some of those biscuits you like."
John nods. An uncomfortable silence hangs between them.
"Thank you," John says quietly, looking Sherlock in the eye.
Sherlock nods. "Er. Yes. Well, I, er-"
"I'm not going to pretend to understand," John adds. "But if you think that this is best I'll trust you for now."
Sherlock nods again and smiles and he and John share a deep look.
"Oh, Lestrade called," John says, "There's been a murder in Kensington. Interested?"
Sherlock grins, and John sees his eyes sparkle. "Always."