More than a decade in the making, America's opioid crisis has morphed from being driven by prescription drugs to one fuelled by heroin and, increasingly, fentanyl. Drawing on historical lessons of the era of National Alcohol Prohibition highlights the unintended, but predictable impact of supply-side interventions on the dynamics of illicit drug markets. Under the Iron Law of Prohibition, efforts to interrupt and suppress the illicit drug supply produce economic and logistical pressures favouring ever-more compact substitutes. This iatrogenic progression towards increasingly potent illicit drugs can be curtailed only through evidence-based harm reduction and demand reduction policies that acknowledge the structural determinants of health.