Moving back in was the easy part, in fact Mycroft practically flung his things through the door eager to get his brother a much needed personal doctor. The hard part was the unwilling patient.
The first thing John did, right after Sherlock had retreated to his bedroom, was to search the flat. He looked in all the usual places, except for Sherlock's room of course because conducting a drug search with him present would surely call for another violent reaction, and John's back still hurt from the thrashing he'd received.
Even with Sherlock's mind slipping as it was, he was still as clever as always. John's search became painstakingly difficult, and a few hours had passed by the time he finally turned up a few bags of the white powder.
John's back was aching, his head was pounding, and he could feel that old familiar pain in his leg starting again. He decided to just take a moment to sit down and gather his thoughts.
"How do I fix this...?" he sighed to himself. If he could just approach this like Sherlock did...the way he always just sat down and thought for hours on end and would then leap up with his eyes gleaming and a triumphant grin on his face. What he wouldn't give to have that Sherlock back. This Sherlock, this raving and delusional Sherlock with ill eyes, scared him. It must scare Sherlock too, to have clouded his own mind so much.
"Alright. First I need to eliminate any stash of the drug he has. That's half done." John said to himself. "Then I need to break the addiction...then I need to find out why he started again to prevent it from happening another time."
What could have caused Sherlock to turn back to cocaine? Sherlock had told him of his younger years when he'd turned to drugs because of "family troubles" as he referred to them. When John had asked Mycroft what that meant, the elder Holmes brother had told him that if Sherlock wanted him to know he'd tell him in his own time. Clearly he'd gotten over it, so why was he using again now? He could practically hear Sherlock's voice in his head...
Think, John, think. It's so obvious, staring you right in the face really. What kind of idiot wouldn't notice it?
He smirked. This was the Sherlock he needed back...

"Get away!" Sherlock lobbed the nearest object, it happened to be a mug, at John trying to fend him off.
"Sherlock! Stop this!" John took a deep breath, you couldn't yell at patients you had to be well...patient with them. He needed to calm Sherlock down, get him to sleep.
The detective had woken in the night in the grip of a paranoid nightmare, when John came downstairs to investigate he'd been attacked...again.
"Sherlock, it's just me, just let me help you." John put his hands out, and Sherlock's eyes seemed to clear up a bit, but his shivering continued. "Just let me help..." He had to keep him still before he hurt himself.
Or me. John thought wryly. I just need to keep him still...
He managed to get close enough to Sherlock to touch him, he moved slowly like you would with a wild animal.
"I won't hurt you. I'll keep you safe I promise..." John pulled Sherlock into his arms and held him there tightly.
"John..." Sherlock's breath was hot on John's ear. The detective whimpered slightly as John lifted him up and placed him back on the bed. His pale fingers found purchase on John's jumper and he pulled him in tighter. John allowed himself to be pulled down onto the bed, and wrapped Sherlock up in his arms.
Just to keep him still of course. No other reason. Clearly.
"There now...you're alright." John whispered.
"...I know John...I'd ask you not to patronize me but I deserve no better...my mind seems so clouded..." Sherlock, in a moment of rare clarity, replied.
"I'll fix that." John said, and before he knew what he was doing he leaned in and kissed Sherlock on the forehead.
John held his breath, his cheeks flushed with red he sat and waited for the cutting remark Sherlock would throw at him yet none came.

On the fourth day, Sherlock displayed clear signs of being well on the way to recovery. Even going so far as to make sarcastic remarks whenever John tried to note withdrawal symptoms. Most of it seemed to be to save his pride, after all when the cleverest man in the room is dissolved into a paranoid wreck, what else can he do but act even more clever to cover his shame? So John let him get away with more than a few rude comments, simply taking his pulse and checking his pupils in silence. It was during one of these instances that Sherlock revealed his secret.
"My mother." he said simply. He was sitting on the couch with John kneeling in front of him, the doctor had already swept the apartment for drugs and checked Sherlock's pupils in what Sherlock called "an exercise in futility seeing as I am fully cured".
"What about her?" John asked distractedly, peering at puncture marks trying to figure out if they were old scars or new wounds.
"Nothing much really...she was a pleasant woman..." Sherlock pretended to find something of great interest outside the window. Then a few moments later: "My father killed her."
John froze, he let the last sentence register and then looked up at the detective who was refusing to make eye contact.
"She had a lover." he continued. "She'd had one for quite some time, it was obvious to me and Mycroft of course. When father found out he was furious. So he killed her, and then himself." Sherlock's bright eyes slid around and found John's, which were full of concern. "It's why I started using cocaine. Or at least the closet reason I can think of. My parents were gone, Mycroft grew colder, I was...lonely. After that it was just because the thrill of the drug was...irresistible." Sherlock swallowed.
"...I'm sorry." John murmured.
"Oh don't be stupid, John, I wasn't telling you to get pity." Sherlock huffed.
"...Why did you start using this time?" John pushed, still curious. Sherlock stared down at him blankly.
"I was bored. I was lonely."
Oh.
"John left."
" Think, John, think. It's so obvious, staring you right in the face really. What kind of idiot wouldn't notice it?"
Oh!
"S-Sherlock..." John stuttered, but Sherlock just looked away.
"Like I said before. I didn't tell you to get pity, and I won't have anymore pity from this point..." Sherlock didn't get to finish his sentence because John pulled his face down gently and placed a chaste kiss on his lips. Sherlock blushed, his eyes asking a million questions at once.
"Sherlock, you're hopeless, but I trust you with my life and I can bring you onto a battlefield without worrying about you breaking down." John gave a small grin. "Minus the breakdown we just witnessed of course...still I could never have a healthy relationship with any other woman with the way I live. It had to be you, and I wouldn't have it any other way."

Thanks for reading this far, I enjoy feedback.

I really didn't mean for this story to get so long hence the two parts, but I hope it was good enough to merit it's length. The one-shots will continue henceforth.

Also, if you guys enjoy druggy Sherlock "feels" I highly recommend The Seven Per-Cent Solution by Nicholas Meyer. Great read.