If Maine votes to legalize marijuana, even if only for adults, the biggest problem is that more teenagers will use it. We know this because that’s what happened when laws were loosened to allow medical marijuana. In 1999, before Maine became one of the first states to legalize medical marijuana, it ranked 28 th in teen use; seven years later, it was No. 1.

While decriminalization only eliminates penalties for possession, medical marijuana acts more like legalization. It becomes legal to sell and even advertise. A study published in the journal Psychology of Addictive Behaviors found that teens who saw medical marijuana ads were twice as likely to try the drug. As the lead researcher noted, “Advertising typically tells only one side of the of the story.” Another recent study, from the International Journal of Drug Policy, confirms teenage pot use increases after states pass medical laws.

A huge study, the National Survey on Drug Use and Health, shows that between 2005 and 2011, teen use increased by 33 percent in states with medical marijuana laws, but only increased 6 percent everywhere else. Only 10 states had medical marijuana in 2005, and they accounted for only 20 percent of the U.S. population, but they were responsible for 60 percent of the increase in teen use.

The reason this spells trouble is that marijuana damages the developing adolescent brain. According to an article in last year’s Current Addiction Reports, dozens of structural changes show up on brain scans of teens who smoke pot. These changes are linked to impulsivity, poor attention, worse memory and trouble thinking and planning. And the damage is permanent; even if they quit using, brain function never returns to normal.

The most serious consequence is an average loss of eight IQ points in addicted users who start as adolescents. (Critics of this study said it didn’t account for several other possible causes of lower IQ, so the researchers re-ran the data and still found that teenage marijuana use lowers IQ.) That’s enough to mean someone born to do well in school is instead struggling, or someone who should have been promoted at work is instead passed over.

Other research shows that high school and college students who use marijuana skip more classes, study less, earn lower grades and drop out at much higher rates. No parent wants this.

Fortunately, teenage use is not inevitable. Monitoring the Future, a large annual survey done by the University of Michigan, found adolescent marijuana use went way up when they thought the drug was safe, and way down when they saw it as harmful. Teen use peaked in 1978, but then parent groups began pressuring the news and entertainment media to stop glamorizing marijuana and start showing its downside. Over the next 14 years, daily teenage use dropped from 11 percent of all teens to 2 percent.

This means we can prevent almost all teenage marijuana use — if we stop making the drug sound harmless. But the First Amendment protects speech, so the only way to prevent advertising is to keep pot illegal.

Criminalizing drug use might seem unfair, but it’s prevention, not punishment. Sweden has extremely tough marijuana laws, but offenders usually get probation and treatment, not jail. They’ve even closed four prisons over the past decade. The problem of lifelong stigma for a possession arrest can be remedied with expungement; in several states, after a few years the person can have his criminal record erased.

By the way, medical marijuana is mostly recreational. A study in California found the average age of marijuana patients was 32, most were male, and 90 percent had been smoking pot since their teens. The few genuine medical users could take prescription cannabinoids like Marinol and Cesamet that work just as well. No one needs to smoke marijuana.

Here’s the bottom line: In Sweden, where there’s not even medical use, a 2013 UNICEF report said only 5 percent of teens under age 16 smoked pot. In the United States, the rate was 22 percent. And in Colorado, teen use is 50 percent higher than the national average.

There is no way to legalize a drug for adults without also increasing teenage use. That’s the lesson of alcohol, tobacco and now medical marijuana. If we want to protect the next generation, we should keep marijuana illegal — not for punishment, but for prevention.

Dr. Ed Gogek is an addiction psychiatrist in Prescott, Arizona, and author of “Marijuana Debunked: A handbook for parents, pundits and politicians who want to know the case against legalization.” He was a resident in psychiatry at Portland’s Maine Medical Center in 1985.