It didn't start out this way. A cigarette here, a missed meal there. It wasn't until one night, after a difficult case, that John noticed the beginnings of a problem.
"Sherlock, you gonna touch that egg roll?" John muttered through a mouthful of greasy pork fried rice. The chopsticks lay abandoned in the paper takeaway bag, along with the tiny packets of sauces. The detective rolled his eyes, pushing his barely touched plate toward his flatmate.
With short, clipped movements, Sherlock pulled a scratched cigarette case from his pocket and lit up. It took John all of a second to pull it away and snub it into the remains of the Szechuan pork. "Oi, you great git, I live here too." John's eyes narrowed in realization. "You were supposed to have quit."
Sherlock huffed, putting his hand into his pocket as if to shield the case from John's awareness. "You're a doctor, John, you must know the likelihood of taking up the habit increases after each unsuccessful attempt to—"
"Show me your arms." John growled and reached over the table to grasp at the tall man's arms. Sherlock's sleeves gave way under his fingertips; no track marks. The detective sucked at his teeth in irritation, plucking at the fabric to right it. His ice blue eyes looked back down to the treatise on herbs sitting within reach. John's eyes softened, and he reached across the table, more carefully this time. "Sherlock, I trust you with my life. I just can't be too careful, yeah?"
As John waited for the man to warm back up to him, he noticed his hand shaking beneath his own. "Hey, you alright, love?"
Sherlock pulled his hand away, his back to John. "I'm off to bed. Goodnight, John."
John sighed, the takeaway soon forgotten. He pushed his chair away from the table, following the detective to the bedroom in silence. The taller man got to the door and turned the knob, budging it open with a foot. John closed the door quietly, watching as his partner shucked his dress shirt and pants, leaving them in a heap on the floor. That was unlike Sherlock, he usually put them neatly away as soon as his clothes left his skin; it was one of his many eccentricities, something that made him Sherlock, and John immediately noticed the change while he tucked himself under the covers, curled into himself.
John picked up the discarded clothes, laying them on top of the hamper with his own. A light from the street shone past the edge of the curtain, illuminating just the curly mop of hair on top of Sherlock's head. John crawled into bed, sidling up to the silent detective and pressing his chest to Sherlock's smooth, pale back. He kissed a beige freckle, just below his nape, and curled a protective arm around the brunet.
"You mind telling me what's wrong?" John whispered, unwilling to break the burgeoning silence in the room. The worn duvet felt soft against John's back, and the room seemed harsh and cold, in comparison. He tangled his feet with Sherlock's, a habit he picked up after months of sharing a bed with the lanky man. His toes barely reached Sherlock's ankles, but he felt cozy, just the same.
Sherlock feigned sleep, until a brush of John's lips over his scapula made him sigh.
"I'm just fine, John. I promise. My body's needs caught up with me, is all. It was a rough case, what with the estranged father's affair, and the daughter's prescription drug smuggling ring—"
"I know, love, I was there, remember?" John smiled fondly. "How did you find out about that, anyway?"
"Her boots. They were far too expensive, when compared to her bag and her cheap haircut. It was obvious she was getting the money from somewhere other than the family's antiques business." John could almost hear the exasperation in Sherlock's tone, the "it-was-hardly-a-stretch-and-you-should-have-known" lilt to his voice, all too familiar.
"Alright, I'll let you sleep, then," John murmured, burying his nose into Sherlock's curls.
All through the night, he felt the occasional twitch in Sherlock's foot, and thought nothing of it. He imagined he was dreaming of the chase, his coat billowing behind him, and John not far behind.