John Watson had been living with Sherlock Holmes long enough to know about "danger nights". These "danger nights" could be caused by anything: boredom, failure on a case, being mistaken once too many times during a day, for example. But what caused the "danger nights" didn't really matter to John. No, why would the cause of them matter when they all brought about the same result? John's father had been a substance abuser with a nasty temper, and passed down the former of the mentioned to John's sister, Harry. It was because of this that John never was a fan of those who used various methods to alter their mood.
It wasn't long after John moved into 221B that he discovered his flatmate used these various methods to influence himself in one way or another. He was a bit shocked, yes (and even a little disappointed, if he was to be honest with himself), but the way Sherlock had insisted he was clean had made his habits sound like they disappeared long ago. John hadn't been exactly wrong on this - Sherlock used to go days without coming down from a high, according to Mycroft - but he wasn't completely right either. It was soon found out by John that his flatmate, whom he admired and respected so very much, wasn't as clean as he claimed.
The first day John came home to what Mycroft later told him was a "danger night" he was exhausted. Work had gone far longer than usual that day, due to a nasty bug that had been going around at the time. John had suspected he was getting a touch of it himself, which was why he'd called off work the day after. John was nearly home to Baker Street when a memory from earlier that morning slammed into his mind. He stood there for nearly a minute, his face looking as if it was going to crumple at any moment with a slight tremor in his lips. Had a passerby been present, they would've thought the poor man had just lost someone close to him. The situation wasn't nearly as tragic, although John most certainly thought it was, at the moment.
John had forgotten the milk. The stupid, damned milk. The snow that fell around him shone white as the milk he'd forgotten, seemingly mocking him. Most people in John's shoes would've shrugged it off and headed home, but not John. He had made such a fuss over not having milk that morning that Sherlock would definitely mock him for not picking any up.
John had closed his eyes and let out a deep breath, steeled his resolve, before he'd turned around to walk five blocks back to the mart to buy the damned milk. Thirty minutes and small tussle with a small Yorkie on leash later, John was on his way back home. Needless to say he was not in his cheeriest mood when he finally entered 221B Baker Street. It wasn't uncommon to not see Sherlock when first entering the flat; the man could blend into shadows if he wanted to. What was unusual was not to have Sherlock make any sort of noise within twenty minutes of John arriving home. John, being as tired as he was, didn't think much of Sherlock's unusual silence until he was sitting by the fire with a book and his legs stretched before him. Sherlock typically didn't waste his breathe with greetings, even for John, but he could nearly always he heard. A slight rustle of fabric, a bored sigh, the popping of bones after bending over a microscope for hours. Those noises were ever present with Sherlock in the house. The man could be silent when he wanted, but as comfortable as he was living with John, the precautions weren't necessary.
So to not hear any sound of Sherlock was something that unsettled John a bit. He had glanced towards the door where their coat hangers were only to see Sherlock's coat still resting there. His shoes were tucked surprisingly neat beneath it. John felt even more unnerved at that, but tried to reason with himself. Sherlock may only be napping, he thought, but had dismissed the idea immediately. Since when did Sherlock nap?
"Sherlock," he'd called out. When there was no reply he rose a bit unsteadily to his feet, his tired feet protesting. He'd strode to Sherlock's room and rapped gently at the door. "Sherlock?" he'd said again, leaning slightly on the door's handle. The door gave, and John stumbled a few feet into the room with an apology ready, just incase he had awoken his sleeping friend.
Sherlock hadn't been asleep, though. He had been sprawled across the ground in his darkened room, wearing his ratty pajama pants and shirt. His left arm was stretched into the air, his pale eyes studied it with the intensity he usually saved for cases.
John knew better than nearly anyone that Sherlock tended to stray from normal human habits, and honestly lying on the ground looking at his arm wasn't the strangest thing John had ever seen him do. But something was off, something wasn't quite right with Sherlock demeanor.
"Sherlock, what are yo-...?" he'd started, shuffling into the semi-dark room as he spoke. But by then his eyes had adjusted better and he could make out a smallish object by Sherlock's right hand as well as an tiny red spot on Sherlock's left forearm.
John had blinked several times looking back and forth between the empty syringe and his flatmate. It was then that Sherlock lazily turned his head to John. His pupils were overblown and his smile wasn't the smile John had come to associate with Sherlock Holmes.
"Problem?" Sherlock had dragged out, letting his left arm fall to the carpet. John hadn't been able to form any coherent thoughts, let alone an answer.
In the end, John wasn't sure how he got out of Sherlock's room or how he ended up dialing Mycroft for advice. Sherlock didn't really forgive him for going to Mycroft, but to John the occasional glares he received from the detective were completely worth the aid Mycroft gave him. John had already lost his father and sister to substance abuse, and he'll be damned if he allows Sherlock to continue what he's doing, regardless of how far between his doses are. So no matter what the plans John has, he'll cancel them if Mycroft calls a "danger night". Despite what anyone else might think, John believes Sherlock deserves to have something besides drugs to keep him company on "danger nights".