Another legal drug, nicotine, kills more people than do alcohol and all illegal drugs — combined. For decades, government has aggressively publicized the health risks of smoking and made it unfashionable, stigmatized, expensive and inconvenient. Yet 20 percent of every rising American generation becomes addicted to nicotine.

So, suppose cocaine or heroin were legalized and marketed as cigarettes and alcohol are. And suppose the level of addiction were to replicate the 7 percent of adults suffering from alcohol abuse or dependency. That would be a public health disaster. As the late James Q. Wilson said, nicotine shortens life, cocaine debases it.

Still, because the costs of prohibition — interdiction, mass incarceration, etc. — are staggeringly high, some people say, “Let’s just try legalization for a while.” Society is not, however, like a controlled laboratory; in society, experiments that produce disappointing or unexpected results cannot be tidily reversed.