Just in case I forgot to properly define my term "pill mill":
en.wikipedia.org
The distinctive thing about them (that makes them worth looking into), is that they are run by medical professionals, who prescribe legitimate and safe drugs, made by pharmaceutical chemical companies in regulated and carefully maintained factories.
The risk is people who wouldn't otherwise get addicted to opioids would. The benefit is that once you have an addict, they are able to easily get their fix from medical professionals who only prescribe legit, safe drugs. So the chances that they overdose is much lower than now, when many of them can only get their biochemically motivated fix from street drug dealers dealing in street drugs that are 'laced' with fatally potent fentanyl.
This proposal, the OP, isn't about reducing opioid addiction, it's about saving the lives of people who find themselves opioid addicted, however that occurred. It could very well result in more addicts, but all things considered, it would save more lives than our current situation, where 'pill mills' are hounded out of existence by police.