avawatson said: 

I could be missing something but — I thought he didn’t actually use. He denied using and then got tested by Molly — clean. Maybe I missed something in between, but why do you think he actually used?

You totally missed something, I’m afraid. ^_^  Molly got a POSITIVE test on the drugs being in Sherlock’s system (that’s why she slapped him), and Sherlock himself admitted to Mycroft that he was high when Mycroft visited the flat afterwards.