This coming Tuesday, nine states will consider ballot initiatives that legalize marijuana for medical or recreational purposes under state law. Twenty-five states have already legalized marijuana for medical use, and four have legalized fully, and polls suggest many or most of the new initiatives will pass. Opponents nevertheless make strong claims about adverse consequences from existing and proposed legalizations. We argue, based on the evidence, that such claims are exaggerated, misleading, or outright false.
Legalizing marijuana dramatically increases use: Several countries (Portugal, the Netherlands, Australia, and part of the U.K.) have liberalized their marijuana laws with little or no impact on marijuana use. Research on U.S. medical marijuana laws suggests that adult marijuana use has increased only modestly. Preliminary data in Colorado and Washington, the two first states to legalize recreational marijuana, display similar trends in use before and after legalization.
Legalizing marijuana increases other substance use: Whether legalization affects other substance use depends on whether new consumers progress to drugs such as cocaine or heroin (the gateway effect) and whether existing consumers substitute marijuana for other substances. No scientifically convincing evidence supports the gateway hypothesis for marijuana. In fact, some research suggests that users substitute from alcohol toward marijuana after liberalization. Rates of cocaine use appear unchanged in the wake of recreational marijuana laws. Research on medical marijuana laws shows little impact on alcohol or cocaine use.