If illicit drugs are found in his body, someone will talk about what happened and whether or not it was always going on or new. I mean, it sounds like TMZ is convinced he had relapsed recently, but reading between the lines I think their sources are the other musicians who got the alleged 'drunk texts.'
I am guessing if it were on OD we would have an indication already. My guess is heart failure. Everything he was doing (alcohol, illicit drugs, smoking) wreaks havoc on your blood pressure and most of it can enlarge your heart over time. Guy was nearly 50. A friend of a friend of mine died at 38 from the same living, plus he was obese. So nearly 50 doesn't surprise me.
It's too bad he couldn't continue replacing his drug high with exercise like he had during No. 4 era and Contraband.
To be perfectly honest, I think Scott's truly sober days were few and far between. I am guessing his release from jail to about the recording of Shangri-la Dee Da (when he got into prescription meds), and then a short time following the release of Contraband. Otherwise, I'd bet he was either drinking, taking prescription meds, doing illicit drugs, or some combination of all of those.
It's possible Scott thought that if he could just drink, stay away from heroin, and try and control the other stuff, he was doing well. But with what he know about addiction, child abuse, and how your surroundings dictate your addictive behavior, the rape he admitted to when he was younger means there was a huge increase in the chances he would become an addict. And as far as his surroundings, more and more doors kept slamming in his face (STP, VR), close people around him left his Earth (Jeremy, his brother, etc.), and he had to continue to live on a bus and travel away from his family in order to afford his lifestyle. That's quite a small cage to live in constantly. The Daily Mail said that one of his shows in an 1,800 seat venue was canceled this leg for selling fewer than 100 tickets. So even his fans were no longer showing up.
Really just sounds like a recipe for disaster. I can't say I blame the guys in STP from cutting ties 100%, but as more and more people cut him off, that probably drove him into more destructive behavior.