I don't have narcissistic parents, but I'm an addict. I don't remember "why" I started, I know I have blamed it on a traumatizing event in my life that happened over 20 years ago, but I'm not so sure that is why. I did a lot of drugs(soft - weed, lsd) before this event, I just started doing hard drugs(crack, opiates) shortly after. I always liked getting high. Maybe this is your brother's reason. Maybe he needs an escape from his life.
The point is, it's not your fault. Even if you introduced him to the drugs, which doesn't seem to be the case, it's his choice to continue to use them. Addicts are looking for people to blame for their shitty lives. Their enablers do the same. Sometimes people just like to get high, like me. I can't speak for all addicts of course.
Your brother is playing a very dangerous game with those oxies. Once the opiate addiction sets in it digs trenches and starts a war. It makes it seem impossible to break free. Years fly by before you know it. What was once use to get high is now use to get normal. Normal being not having goo constantly coming out your nose, not pissing out of your ass, not sweating buckets while shivering cold at the same time, not feeling electrical shocks up and down your body, not having to move every second because your legs and arms just can't stay still, not feeling like your skin is on fire, not feeling every single ache and pain you've ever had. Paying the $20 for an oxy is a lot better than all that.
Then then longer you do it the more you need, just to feel better. What started as doing 10 or 15mg of oxy a day turns into 120mg, 180mg, 300mg, whatever. I have seen people eat handfuls of 80s to get better. Easily $500 worth at a time at street prices.
It's not a life I would wish on my greatest enemy. I don't know your relationship with him. I don't know how to help him. I have tried detox and rehab many times, 12 or 15, I dunno I lost count. If he's just starting with the pills, then the addiction hasn't truly set it. When it starts to set in, it will seem like he has a cold, or a flu. He won't even know he's actually dope sick. 3 days in a row is what i noticed when i first started. If I used 3 days in a row, I would get a cold. Unfortunately there were not the information resources back then like there are now, so I didn't realize what was happening.
He shouldn't do them. I know this is not the point of the post. I'm sorry to go off topic. I'm just here saying that if he continues to use he's going to have a bad time. Quite possibly ruin his life. It won't be your fault of course, like I said we addicts are the only ones who make our decisions. If you notice he has a cold, or is pretty irritable chances are that is the addiction digging in. Maybe a nudge to some websites or something would help him. Or not. I don't know. I just know it sucks being 43 and an addict since I was a teen. I just want to shake every young person I see getting involved with this shit and tell them to stop now. There is no good to be had from it. Overdose, suicide, murdered, jail. That's where all the friends I have had are. I've done a bit of jail time, unrelated to my use, but one of the other items on that list is picking me soon. It's inevitable.