Cannabis—marijuana—is the world's most commonly used illicit drug. Polls suggest that one in eight U.S. adults smoke it, and more than 40 percent of them have tried the drug at some point in their lifetime. A majority of states allow some form of medical marijuana use, and nine states and Washington, D.C., have now legalized recreational use. Although the substance is illegal under U.S. federal law, in 2013 the Justice Department under President Barack Obama guided U.S. attorneys away from prosecuting personal marijuana use in states where it is legal. But in January, Attorney General Jeff Sessions reversed those guidelines, giving U.S. attorneys renewed authority to press criminal charges.

Like the failed Nixon-era War on Drugs, this resurgent war on marijuana is ill informed and misguided. Evidence suggests that cannabis—though not without its risks—is less harmful than legal substances such as alcohol and nicotine. And despite similar marijuana use among blacks and whites, a disproportionate number of blacks are arrested for it. By allowing states to regulate marijuana without federal interference, we can ensure better safety and control while allowing for greater research into its possible harms and benefits.

In 1970 the Controlled Substances Act established marijuana as a Schedule I drug, “with no currently accepted medical use and a high potential for abuse.” This is the same category that includes heroin and MDMA (ecstasy). Yet marijuana is far less dangerous than many other drugs, and cannabis or its derivatives have been used to treat everything from chronic pain to post-traumatic stress disorder to childhood epilepsy. A 2015 study that compared the toxicological threshold of marijuana for risk to human health with that of other drugs found that alcohol posed the highest risk, followed by heroin, cocaine and nicotine. Marijuana was among the lowest. In addition, there is some evidence that pot may serve as a safe alternative to other drugs of abuse, including heroin and other opioids.

That does not mean that marijuana is entirely benign. Studies suggest it can impair driving, and a subset of users develops a form of dependence called marijuana use disorder. Other research indicates that teenage marijuana use may adversely impact the developing brain: it has been linked to changes in neural structure and function, including lower IQ, as well as an increased risk of psychosis in vulnerable individuals. But some of these findings have been challenged. A pair of longitudinal twin studies, for example, found no significant link between marijuana use and IQ. Moreover, people with these brain characteristics may simply be more likely to use marijuana in the first place.

We are not advocating for unfettered access to marijuana, especially by adolescents. More large-scale, randomized controlled studies are needed to tease out the risks and benefits. But to do these kinds of studies, scientists must have access to the drug, and until very recently, the federal government has had a monopoly on growing cannabis for research purposes. We also need more research on the various, often more potent, marijuana strains grown for recreational use. As long as the federal government continues to crack down on state-level legal marijuana, it will be difficult to carry out such studies.

Even those who oppose cannabis use should reconsider the efficacy of criminalizing it. One of the most compelling cases for easing restrictions comes from Portugal, which decriminalized all drugs in 2001. Drug usage has remained the same or decreased as a result, and drug-related deaths and sexually transmitted diseases have dropped significantly. Portugal's experience may not translate directly to the U.S., but its success is worth noting. A 2014 study found that medical marijuana legalization in the U.S. has not increased crime and may actually be linked to lower assault and homicide rates. Even a limited version of federal reform, such as downgrading cannabis to a Schedule II or III drug—categories considered less harmful—could prove beneficial.

It is time to stop treating marijuana like a deadly drug, when science and public opinion agree that it is relatively safe for adult recreational use. The last thing we need is another expensive and ineffective war on a substance like cannabis—especially when there are far more serious drug problems to tackle.