It depends on the person and how they're wired. Plenty of people have taken pain killers post surgery or whatever and didn't get addicted. Other people get hooked almost immediately.
I've popped pills before. I've smoked heroin a couple of times with people. It was quite enjoyable but it didn't become a habit for me. Same with cocaine. I actually have learned that I don't care for that drug very much. Plenty of people can use cocaine socially and not get addicted, but other people can't go near the stuff.
My habits are cannabis, nicotine, and caffeine. That's just the way I'm wired. I also enjoy beer a little too much and I'm always trying to cut back.
I've known functional pot heads, functional alcoholics, functional opioid addicts, functional meth users...
Not everybody is wired the same. But you have to let adults make their own choices about their own bodies and health.