Can we please stop pretending marijuana is harmless? The view from New England.

These days, it’s become fairly square to criticize marijuana and its rush toward legalization. Twenty-three states have condoned the drug in some form, with four permitting recreational use, and Massachusetts is set to vote on permitting it next year. The proposed federal CARERS Act of 2015 would let states legalize medical marijuana without federal interference and demote pot from a Schedule I drug — one with high abuse potential — to Schedule II. The path toward nationwide decriminalization is looking unobstructed.

But underscoring the incredible momentum to legalize marijuana is the misconception that the drug can’t hurt anybody. It can, especially young people.

The myth that marijuana is not habit-forming is constantly challenged by physicians. “There’s no question at all that marijuana is addictive,” Dr. Sharon Levy tells me. She is the director of the Adolescent Substance Abuse Program at Boston Children’s Hospital, one of a few programs designed to preemptively identify substance use problems in teens. At least 1 in 11 young adults who begin smoking will develop an addiction to marijuana, even more among those who use the more potent products that are entering the market.

Levy speaks of an 18-year-old patient who had started smoking marijuana several times a day in 10th grade, dropped out of high school, and been stealing money from her parents. “She and her family were at their wits’ end trying to find appropriate treatment in a health care system that doesn’t consider addiction to marijuana a serious problem,” Levy says. “We are simply not prepared for the fallout of marijuana legalization.”

Such perspectives have been obfuscated by those who might gain from legalization. “People strongly defend marijuana because they don’t want legalization to be derailed,” says Jodi Gilman, an assistant professor at Harvard Medical School with the Center for Addiction Medicine.

An insistence on the banality of the drug is especially dangerous among younger smokers, a population with an epidemic level of pot use. According to the most recent National Survey on Drug Use and Health, the use of tobacco and alcohol among 12-to-17-year-olds has fallen in the past year, but habitual use of marijuana among those 12 and up is increasing.

“If you go into a high school and ask the classroom, ‘Are cigarettes harmful? Is alcohol harmful?’ every kid raises their hands,” Gilman says. “But if I ask, ‘Is marijuana harmful?’ not a hand goes up.”

To bring balance to a narrative driven by pro-legalization campaigns, Gilman and others are interested in leveraging data to show pot’s real effects. Last year, Gilman published research on 18-to-25-year-olds that showed differences in the brain’s reward system between users and non-users. (“I got a lot of hate mail after that,” Gilman says.) And data supporting the hazards keep accumulating. Recently Gilman found that in a group of college students, smokers had impaired working memory even when not acutely high.

Physician concern for marijuana’s acceptance isn’t because doctors are a stodgy bunch — their skepticism is rooted in science and in history. In the 1950s, nearly half of Americans smoked tobacco, a level of adoption that rendered its health hazards invisible. Meanwhile, the corporate forces that drove cigarette smoking to its ascendancy actively subverted those that governed public health.

While marijuana has not been definitively shown to cause cancer or heart disease, its harmful cognitive and psychological effects will take time to capture in studies. The underlying biochemistry at work suggests deeply pathologic consequences. Tetrahydrocannabinol (THC) in marijuana attaches to receptors in the brain that subtly modulate systems ordinarily involved in healthy behaviors like eating, learning, and forming relationships. But THCwhich has been increasing in potency in legal products being sold in places like Coloradothrows the finely tuned system off balance. 

“Smoking pot turns the volume on this system way, way up,” says Jonathan Long, a research fellow at the Dana-Farber Cancer Institute.

Each hit of THC rewires the function of this critical cognitive system: Early evidence in mice has shown that repeated exposure to THC causes these receptors to disappear altogether, blunting the natural response to positive behaviors and requiring higher doses to achieve the same effect. Marijuana exploits essential pathways we’ve evolved to retrieve a memory, to delicately regulate our metabolism, and to derive happiness from everyday life.

Medical science at its best operates independently of forces that drive the market and its associated politics. It was science that eventually curtailed the power of Big Tobacco and prevented nearly 800,000 cancer deaths in the United States between 1975 and 2000. As marijuana marches toward the same legal status as cigarettes, its potential hazards will require equal attention by science.

The argument here isn’t whether marijuana should be legal. There are champions on either side of that debate. Instead, should the drug become widely available, it’s to our detriment to blindly consider marijuana’s legalization a victory worthy of celebration. We must be cautious when societal shifts can affect health, especially among our most vulnerable populations.