I don't think cutting off supply is really possible, or practicable. It's too easy to smuggle. And it's making bad actors very wealthy.
Fentanyl and major fentanyl analogs. Fentanyl is an extremely powerful analgesic, 50–100-times more potent than morphine. Its LD50 is of 3.1 mg/kg in rats and 0.03 mg/kg in monkeys. The LD50 in humans is unknown.
If we figure 1 mg/kg in humans, and an average human weighs say 80 kg, geesh, it's a smidgen.