That bag may be legal, but it's not good for you. Photographer: Stephen Brashear/Getty Images
That bag may be legal, but it's not good for you. Photographer: Stephen Brashear/Getty Images

Marijuana has now been legalized or decriminalized in 17 states and the District of Columbia, with Maryland joining the list just last week. Not to harsh anyone's mellow, but it may be an appropriate time (and day) to bring back another useful verb to associate with marijuana use: stigmatize.

The drive toward legitimization will be hard to stop. Most Americans favor it, and ballot measures to loosen rules on marijuana use could come to a vote this year in at least five states. Twenty-one states already allow marijuana for medical use.

What's unhealthy about this trend is that it coincides with a declining awareness of marijuana's dangers -- especially among young people. Less than 40 percent of high school seniors think marijuana use poses a great risk, down from 55 percent in 2003. Cigarettes are dangerous, more and more adolescents have come to realize, but they don't believe marijuana is. (In fact, they're both unhealthy.)

That they could be so wrong about a drug that more than a third have used makes it clear: In their drive to roll back laws against marijuana, and for the revenue that undoing prohibition would raise, states are inadvertently stoking a serious public health problem.

Marijuana poses the greatest threat to the still-developing brains of teenagers. Steady use can bring lasting impairments in memory, intellectual functioning and emotion control. Marijuana use has been linked to depression, anxiety, even psychosis. Smoking pot once a week or more appears to actually change the size and shape of certain brain regions in young people.

Dependence is a special problem, not limited to adolescents but more prevalent among them: One in 6 teenagers become addicted to marijuana, according to the National Institute on Drug Abuse (the comparable figure for adults is 1 in 11). With dependence comes every sort of social trouble: isolation, failure at school and work, often profound unhappiness.

There are other effects, equally disturbing. Smoking pot is bad for the lungs. It slows reaction time (fatal car accidents involving drivers testing positive for marijuana tripled in the U.S. from 1999 to 2010). Pot use during pregnancy can harm the fetal brain, and there remain unanswered questions about how marijuana affects adult and geriatric brains.

When marijuana use was illegal, fears about these effects were circumscribed. Legalization both eliminates the possibility of penalty, encouraging many more people to try marijuana, and lowers the price, making it easier for everyday users to keep their habit going.

Both Colorado and Washington, the two states that have legalized marijuana for adults, have rules to keep minors away from it. Sales to them are punishable by steep fines and jail terms. Grownups aren't allowed to use marijuana in public view. And there are various limits on advertising. Nevertheless, more teenagers in these states are expected to use marijuana than did before it was legalized.

Such restrictions are essential, but they do nothing to educate kids or their parents about the risks marijuana poses to still-growing brains, or to inform adult users about the dangers of overuse. The states should direct tax revenue from marijuana sales toward public education campaigns, as Governor John Hickenlooper wants to do in Colorado. Hickenlooper would also spend some of the money on research into marijuana's effects on pregnant women.

Colorado collected $2 million in January alone. As with all sin taxes, states will have to balance the competing goals of raising revenue and affecting behavior. It is not hypocritical to use money from taxing a product to discourage its use; states do it now with cigarettes and alcohol, for example.

People argue marijuana is no worse than alcohol, which has been legal for decades, and that it has medical uses in treating pain and nausea. But those facts do nothing to lessen marijuana's risks; they only boost the misimpression that the drug is nothing for anyone to worry about.

Marijuana, like alcohol, must be used and sold responsibly. As states make it easier for the public to get marijuana, they are obligated to protect the public from its harms.

To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at davidshipley@bloomberg.net.