Over the past 30 years, a shrewd and expensive lobbying campaign has made Americans more tolerant of marijuana. In November 2018, Michigan became the 10th state to legalize recreational cannabis use; New Jersey and others may soon follow. Already, more than 200 million Americans live in states that have legalized marijuana for medical or recreational use. Yet even as marijuana use has become more socially acceptable, psychiatrists and epidemiologists have reached a consensus that it presents more serious risks than most people realize.

Contrary...