Pot is always good for a giggle, and that makes it hard to take marijuana seriously. The news and entertainment media couldn’t resist puns on “Rocky Mountain high” when Colorado started the year with legal sales of marijuana for recreational purposes. TV stations across the country featured chuckling coverage of long lines outside Denver’s new state-licensed pot shops.

Legalizing marijuana isn’t just amusing. It’s increasingly popular with legislators and the public. And why not? No matter how high stoners get, they’re nowhere near as scary as out-of-control boozers, right? Stoners don’t brawl in bars. They’re not into domestic violence.

A Gallup poll last year found 58% of Americans favoring legalization (although other surveys report more slender majorities). Decriminalization of pot possession is widespread: 20 states sanction marijuana use for medical or quasi-medical reasons, and, following Colorado’s and Washington’s lead, proponents of legalization are targeting Alaska and Oregon for ballot initiatives in the near future, and six other states after that.

Yet marijuana is far from safe, despite the widespread effort to make it seem benign. Pot damages the heart and lungs, increases the incidence of anxiety, depression and schizophrenia, and it can trigger acute psychotic episodes. Many adults appear to be able to use marijuana with relatively little harm, but the same cannot be said of adolescents, who are about twice as likely as adults to become addicted to marijuana. The new Colorado law limits pot sales to people 21 or older, but making marijuana available for recreational use normalizes it in society. The drug will be made more easily available to those under 21, and how long until the age limit is dropped to 18?

Adolescents are vulnerable—and not just to pot. That’s how they are programmed. They make rash and risky choices because their brains aren’t fully developed. The part of the brain that censors dumb or dangerous behavior is last to come on line (generally not before the mid-20s). Meanwhile, the brain’s pleasure-seeking structures are up and running strong by puberty. When you link adolescent pleasure-seeking and risk-taking to marijuana’s impairment of perception and judgment, it isn’t surprising that a 2004 study of seriously injured drivers in Maryland found half the teens tested positive for pot.

Marijuana impairs learning, judgment and memory—no small matters during the adolescent years—and it can do lasting harm to the brain. Dr. Nora Volkow, director of the National Institute on Drug Abuse, has found that marijuana can damage cognitive function in adolescents by disrupting the normal development of the white-matter that brain cells need to communicate with each other.

Most disturbing is a discovery about marijuana last month at Northwestern University’s Feinberg School of Medicine. Researchers there have found lasting changes in “working memory,” brain structures critical to memory and reasoning. A source of ready recall for basic information, like telephone numbers, and solutions to everyday problems, working memory is also a strong predictor of academic achievement.

Dr. Volkow and most other experts are troubled by changing teen attitudes about marijuana. Barely 40% of adolescents now believe regular use is harmful—down from 80% two decades ago. Teen drinking and cigarette smoking have declined, and their abuse of prescription painkillers has fallen off sharply, but teen marijuana use continues to increase. The University of Michigan’s Monitoring the Future survey last year found that more than 45% of high-school seniors have smoked pot and 6.5% now smoke it daily (a rate that has tripled over the past two decades). At the substance-abuse programs of Phoenix House, and at similar programs across the country, marijuana is the primary drug of abuse for close to 70% of teens in treatment.

No one can say how marijuana legalization will play out. A perception of legal marijuana as safe, combined with sophisticated marketing, may well double or triple pot use. Warning of aggressive promotion, drug-policy expert Mark Kleiman, who studied potential issues of a legal marijuana market for the Seattle City Council, pointed out last year: “The only way to sell a lot of pot is to create a lot of potheads.”

As we learn more about the realities of legalizing recreational marijuana, I suspect pot won’t seem so funny anymore. Remember, drunks used to be good for a laugh too. A hiccupping, stumbling tosspot was a staple of Hollywood’s screwball comedies in the 1930s and ’40s. Smoking cigarettes was considered cool. The reality of wrecked lives and ruined health eventually changed public perceptions of these addictions. Now marijuana is becoming more widely regarded as a harmless amusement. That’s not funny, it’s tragic.

Dr. Rosenthal is a child psychiatrist and the founder of the nonprofit substance-abuse treatment and prevention organization Phoenix House.