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1. Prescription drugs became more effective in pain management.
2. Pain management became more common.
3. Addiction to pain medication became an issue.
4. Pain medication alone didnt provide the high anymore.
5. Heroin and fentanyl become the drug of choice to get to rhe higher high.
I'm not so sure on the accuracy of 1 & 2.
I've been prescribed pain medication in the past, and took only the minimum necessary. I didn't enjoy the slight "high" feeling that came along with it. I was also prescribed same by a dentist, and though I had the prescription filled, didn't take even one. My son just had minor surgery and was prescribed pain killers, he was scared to take it. Eventually, he took one, but then felt he didn't need any more. I would guess that most people won't sell or give away any leftovers they have (I probably have quite a bit of leftovers at home, which I keep in case of emergency).
Doctors are very careful and hesitant in prescribing these nowadays, much more than 5 or 10 years ago. And there's a lot more oversight. I honestly doubt whether painkiller are the gateway (except when some really needed them for some time, and got addicted - which isn't common enough to make this into an epidemic).
Now weed is a totally different story. I can see how legalization, removal of stigma as something totally bad, etc. etc. could make that a very easy gateway.