But the legalization of marijuana presents a natural experiment that allows us to test the gateway-drug argument. If anything, it looks like the opposite is true. In states that legalized marijuana for medicinal or recreational purposes, opioid prescriptions fell substantially. Opioid overdoses fell too. In Colorado, marijuana legalization was followed by a drop in teen abuse of heroin. Opiate overdoses, which had been climbing steadily in Colorado, suddenly began to fall after cannabis became legal.

Instead of a gateway drug, marijuana looks like it’s a substitute for more addictive, more toxic substances. At a time when the U.S. is suffering a devastating epidemic of opioid and heroin abuse, marijuana’s use as a substitute for these harder drugs is much needed.

Another fear was that legal marijuana would lead to an increase in criminality. But a team of economists found that liberalization of state marijuana laws led to no increase in youth criminal behavior.