Sorry in advance for this…
Reichenbach spoilers.
John knew he needed to clean out the flat because who knows what rotting limbs are left over from Sherlock's experiments. But John really didn't want to. He didn't want to go back; he didn't want to smell Sherlock or think about again what his last words in person were to his best friend.
But John cleaned it anyway because he just couldn't make Mrs. Hudson do it.
Hours after he started, everything Sherlock ever touched was sealed away tightly in various boxes. John bent down to touch his toes to give his back a stretch. As he bent down, he noticed a scratched floorboard at the very corner of the room. It had been covered with files and books and Petri dishes full of anything and everything, so John never noticed it before.
He lifted the floorboard and sat back as he looked through the needles and rubber strips for stopping circulation and vials and vials of Sherlock's cocaine concoction. John knew about his past drug habits, but he thought that Sherlock was clean finally. He thought he found all he could in Sherlock's sock drawer that one day.
Lestrade explained the effects of Sherlock's drug once, and they didn't seem harmful to John…
So John took a new needle, injected guiltily, sadly, and saw that his future was bleak.