It's basically impossible to keep drugs out of prison.
I couldn't figure out how to do it. If there's one thing you need to know about junkies is that they want their drugs and they will get their drugs.
The War on Drugs was declared before I was born yet I still see people leaning out in public, after having just shot up if I go into the wrong area.
The only real answer is harm reduction, and I don't mean harm reduction in the form of fucking safe-injection clinics, I mean harm reduction in the form of readily available opium.
You'll never get a junkie who doesn't want to quit, to quit. They have to reach that conclusion on their own. What you can do is crack down on the synthetics and make the milder form available. At least that doesn't eat up government resources because "all the junkies are overdosing because the latest synthetic is way too powerful so we should drop a ton of government money on giving them the best healthcare in the country."
Junkies will be junkies, and the harshest measures imaginable have just made the problem worse. I'm sick of paying to keep them alive and concentrating them around safe injection sites (right downtown,) the fentanyl stream needs to stop and it profits far worse regimes than even Afghanistan.