“Those laws create a black market in which the composition and potency of drugs is uncertain and highly variable. They also push traffickers toward highly potent drugs such as fentanyl, which are easier to conceal and smuggle. As a result, drug users like Gentili typically don’t know exactly what they are consuming, which magnifies the risk of a fatal mistake. The “poisoning” that Peace and Caban decried therefore is a consequence of the policies they were proudly enforcing in this very case.”