Fentanyl is popular because it so cheap (a large Asian country produces it cheaply). If it was taxed anywhere near the damage it causes, it would be too expensive to buy, and smuggling it in via Mexico would still occur.
Make other aspects expensive. Drug crimes and related crimes get huge penalties. No medical treatment if you have certain drugs in your system. Make it so the cost of using fentanyl is so high that there is some deterrence.
