Speaking as an ex-user, opioids are a horrendous class of drug. They steal your motivations, your desires, your dreams—your very soul. They turn you into a shell of your former self.
Do not fool yourself into thinking that opioids are gentle and harmless just because they are technically non-toxic.
When I said "gentle" and "harmless" I meant their psychical properties to human body. A roulette or deck of cards is also harmless to body, but they can ruin lives too.
But the main tragedy is when people lose health, money, commit crimes, prostitute themselves, etc. An as heroin is relative safe and harmless to body, legal heroin supplies to addicts would solve so much problems.
let's say you can place everything you've felt in life on a scale of good to bad feelings from -10 to 10. shooting heroin is like suddenly having an experience rated at 100 on that same scale. the entire scale has permanently shifted. everything else in your life has completely paled in comparison to that experience whether you like it or not. you will have to deal with that changed perspective for the rest of your life.
a good 30-40% of my friends were addicts and from talking to them i think they'd all agree that it's a long, slow climb back to emotional normalcy. i think it's healed only by time. a lot of time. and i think it's a half-life sort of curve where you get 50% better after the first 5 years, then 25% better after the next 5, then 12% better.. etc. more than anything you're trying to reorient yourself towards a life where the joys are small and fleeting, and waiting for those potent memories of having your bliss-button chemically held down to fade.
Chronic heroin use absolutely does alter brain chemistry very significantly.