Derek knows when it started. He was in New York. Laura dragged him to a party at an omega's apartment. It was crowded, loud, and smoky. Laura told him to socialize and left him for a game of darts. He had spent an hour nursing a beer on the couch when a brunette in a short dress introduced herself. He doesn't remember her name or the names of her friends. They met in the laundry room over a black cloth bundle. Inside was a needle and three vials of Ketamine for each of them. The brunette told him to only take one his first time, to keep the other two for later and come back for more if he liked it.
Laura didn't speak to him the next day. When she finally did, she was yelling. Derek yelled back "We're not human, I can take it!" She looked at him like she didn't know who he was. When they'd both calmed down, Laura explained how even horse tranquilizers can affect werewolves in large enough doses. She told him it wasn't any different from humans taking street drugs.
They still came across K dealers once in a while after that. Derek promised he wouldn't use again, though. Still, Laura didn't introduce him to any more shady omegas. When Laura died, Derek thought about the two expired vials in the trunk of the Camaro. He thought about getting rid of them like he promised Laura. He ended up using the second vial. He tried to do it slowly. Only take it in small doses, only when he needed it.
It wasn't long until the third vial was gone, too. Later, he noticed his hands weren't as steady as they used to be. His body wasn't healing as fast as an alpha's should. So when he overheard Scott mention he has a key to Deaton's office…
Stiles put up a fight; he wanted to know what Derek's "plan" was. Stiles asked too many questions. Like why Scott can't know. Derek resorted to threats. "Alright, alright. I'll get the key and meet you there at eight, okay?"
Stiles notices that Derek has been different somehow, like he's sick. He seems slower and dark circles have been developing under his eyes. But he doesn't mention anything as they sneak into the vet's. Derek opens drawers and cupboards like he's looking for something specific. It seems like he finds it in a cabinet. "Dude, did you just pick that lock with your claw? Whoa what are you doing?"
Derek is taking his shirt off. Which isn't really new or unwelcomed, but Stiles hadn't expected it. What's more surprising is the tourniquet Derek expertly starts on his arm.
"Derek…" Stiles hears his voice crack. He recognizes the label on the little containers. It registers that if it can take down a large animal, it should have some effect on a werewolf. Derek doesn't look up, just fills and inserts the needle. When he releases the band on his arm, Stiles swears he hears a sigh.
"I'm sorry," Derek says. Stiles isn't sure if he's meant to hear it or not.
Stiles feels used. Derek wasn't trusting him over Scott for help with a plan, but to feed an addiction. That feeling fades when he sees Derek start to smile. It isn't the blessed grin of a junkie, but content. It's like he was seeing the pain being pushed aside for an hour. He sits with him against the wall until it starts to wear off.
Neither of them talk about that night again. Stiles would like to say it was only the one time, that the only reason he kept stealing Scott's spare key was because Derek would rip his throat out with his teeth if he didn't… But werewolves can tell when you're lying.