He's pretty much admitted to doing drugs and hitting the sauce hella hard for years. See: this post from 2014 asking goons to come over and do drugs and get drunk with him to celebrate the SA office closing.
Whether those drugs are (let alone were at the time of that post) opioids is up for debate, though he'd be far, FAR from the first to get hooked on pills after a major, invasive surgery.
He definitely does (or did) Ambien recreationally, as confirmed by multiple ex-friends who rolled over on him over the years; Shmorky's ex did, for example; and while she's schizophrenic, she maintained that he uses it even while she was medicated and stable... plus "my ex's boss pops Ambien for fun" is not the sort of detached-from-reality delusion associated with schizophrenia.
Finally, most addicts don't try to advertise their addiction. For some it's easier than for others, but Lowtax is *incredibly* isolated and basically only sees his very immediate family. The only pics of his day to day life come from them, and are easy to keep a lid if he doesn't like what's in them. The kids probably have no idea, or if they do know daddy pops pills, probably accept that it's for "his back". Maplewife ain't saying shit because of the legal stuff on the horizon. Shmorky lives in a trailer in the middle of the woods, as far as I know, and ain't saying shit. Chinkwife found Jesus and probably doesn't feel like sperging about the dude she was married to a decade ago's pill habit (that probably didn't even exist when they were married)... if she even knows. Or cares. So in this instance, absence of evidence is not necessarily evidence of absence.
Personally? Yeah. I think he pops pills. The behavior fits, the medical history fits, etc.