I lean toward amphetamine abuse, which Null alluded to in his weight loss arc. Since Josh no longer mentions weight loss as much, he probably is just using them to keep from getting even fatter.
His behavior on Twitter is less mystifying because Josh is a control freak. He has no power on Twitter, and worse, knows BROWN POOPJEETS are the Twitter mods, so he regularly gets triggered up a storm there and goes back to his Kiwi Farms hugbox seething in rage, only to be triggered again when users point out simple facts.