Just to add onto this, Stamper was a bad alcoholic since the Sleepycabin days at least. He got worse after moving to San Diego. While I can't say definitively he's a drug addict, he's admit to doing several drugs over the years. It's very likely he's doing something sporadically alongside alcohol looking at his deterioration.
Shad is a confirmed adderall addict. He was accused of doing heroin (as was Stamper) but there was never a definite confirmation.
So he is a drug addict. The adderall addiction got ignored while everyone focused on figuring out if he was a complete heroin junkie.