It’s way easier to keep fentanyl out of drugs when you are distributing the drugs.

The government can’t regulate a black market.

The government can’t regulate a black market, but they could stop the charade of illegal drugs, convert the black market into a regular market, and regulate that. Suddenly you get less fentanyl overdoses, better outcomes for addicts (they're not criminals anymore), AND you remove the power of drug cartels (they'll still exist, but will compete with each other through less murder-y means, like any other capitalist company). Plus, stabilize a lot of third world countries where illegal drug production and export is a big part of the economy, which is what allows the cartels to fight and corrupt the state.

Or, just keep doing what they're doing. It's bound to work eventually, right? Drug cartels are definitely growing less powerful every year, the war of drugs is being won, rate of addiction is going down.