The Editorial Board

USA TODAY

As voters in Michigan and North Dakota head to the polls Tuesday to weigh whether marijuana should be legal for recreational use, they don’t have much definitive data to help make this difficult choice.

Although marijuana has been sold legally in Colorado and Washington state since 2014, conclusions about its effect on public health and safety are either limited or mixed.

While opponents predicted that use by teenagers would skyrocket, Colorado high school students' use of marijuana has stayed about the same since 2013, according to state studies. Last year, 19.4 percent reported use in the past 30 days, close to the 19.8 percent who used it nationally.

For those a little older, the story is quite different: Use by adults ages 18 to 25 has increased significantly. In 2015-16, about 32 percent of Coloradans in that age group reported marijuana use — up more than 50 percent since the same federal survey was taken a decade earlier.

That increase, and the fact that 11 percent of Colorado’s ninth-graders use marijuana, are worth weighing before Tuesday’s votes, especially in light of well-documented findings of the drug's detrimental effect on young brains.

OPPOSING VIEW:Pot never should have been illegal

Marijuana exposure before birth, soon after birth and during adolescence can cause long-term changes in the brain, including problems with learning and memory tasks later in life, according to animal research and a growing body of human studies.

The brain is still developing until age 25, and much more needs to be known about pot's effects on young people's brains. A major federal study will track a large sample of young Americans from late childhood to early adulthood over the next decade. But until that’s finished, voters have little to grab hold of on this important issue.

There's also this: Despite supporters' insistence that marijuana doesn’t pose much risk, the drug is addictive for some people. About 9 percent of those who use marijuana will become dependent, rising to 17 percent among those who start in their teens. In 2015, 4 million people in the USA suffered from marijuana use disorder — a significant problem that gets little attention in the marijuana debate, particularly from the moneyed interests promoting legalization.

While legalizing sales remains a tough choice, some changes in the nation’s policies are clearly needed: Harsh criminal penalties for personal use should be abandoned. And medicinal use — already legal in 31 states and up for a vote in Missouri and Utah on Tuesday — makes sense, as long as laws are not so loose that anyone can get a prescription.

Eight states have voted for legalization at the ballot box, and Vermont has done so through its legislature. In six — Alaska, California, Colorado, Nevada, Oregon and Washington — the drug is already on sale. Massachusetts and Maine are expected to join them next year. Vermont allows adults to grow and use small amounts, but does not allow sales.

States are the right laboratories to test whether regulated sales are a viable choice, and Colorado lawmakers showed prescience by requiring that numerous aspects of its experiment be studied and reported each year.

Even so, the findings there and elsewhere leave many open questions. Until the haze clears, voters shouldn't hesitate to nip full legalization in the bud.

If you can't see this reader poll, please refresh your page.