In addition to the scenes the others have mentioned, in 513 when Justin getting Brian out of the loft to go to his stag party, Justin tells him "put on your sluttiest club clothes and bring lots of drugs because we're going out."
So Brian is still using recreational drugs after opening Kinnetik and having cancer, but we hear about it far more than we see it. I think he does slow down quite a bit. He smokes a joint several times but I'm pretty sure that the only time we see him snort or pop something is there at the beginning of 501.
When Justin leaves in 507 he does get belligerently drunk and then he throws himself into the fuck-off with Brandon (clearly pain management) but it does appear that he abuses his body a bit less after he has cancer. I think it's probably a combination of factors: he's running his own business and has to be on top of his game all the time, he no longer takes his health completely for granted, and he's also just growing up (no matter how much he tries to fight it in the middle of S5).
He could have spent the entire time after Justin left him in 507 messed up to dull the pain, but he didn't. He got very drunk the night he went to Ben and Michael's and the fuck-off was definitely using sex as pain-management as well as Brian's pride and vanity about not wanting to let go of his status as top stud, but after Lindsay calls him on it he seems to realize he is being rather pathetic and cuts it out.