Three decades ago, I would have been over the moon to see marijuana legalized. It would have saved me a lot of effort spent trying to avoid detection, constantly looking for places to hide a joint. I smoked throughout my teens and early 20s. During this period, upon landing in a new city, my first order of business was to score a quarter-ounce. The thought of a concert or a vacation without weed was simply too bleak.

Outlook • Perspective Judith Grisel is a professor of psychology and neuroscience at Bucknell University, where she is a practicing neuroscientist.

These days it’s hard to find anybody critical of marijuana. The drug enjoys broad acceptance by most Americans — 63 percent favored ending cannabis prohibition in a recent Quinnipiac poll — and legislators on both sides of the aisle are becoming more likely to endorse than condemn it. After years of loosening restrictions on the state level, there are signs that the federal government could follow suit: In April, Senate Minority Leader Charles E. Schumer (D-N.Y.) became the first leader of either party to support decriminalizing marijuana at the federal level, and President Trump (his attorney general notwithstanding) promised a Republican senator from Colorado that he would protect states that have legalized pot.

Illustration by Hanna Barczyk for The Washington Post

And why not? The drug is widely thought to be either benign or beneficial. Even many of those apathetic toward its potential health benefits are ecstatic about its commercial appeal, whether for personal profit or state tax revenue. Legalization in many cases, and for many reasons, can be a good thing. I’m sympathetic.

But I am also a neuroscientist, and I can see that the story is being oversimplified. The debate around legalization — which often focuses on the history of racist drug laws and their selective enforcement — is astoundingly naive about how the widespread use of pot will affect communities and individuals, particularly teenagers. In our rush to throw open the gate, we might want to pause to consider how well the political movement matches up with the science, which is producing inconveniently alarming studies about what pot does to the adolescent brain.

Marijuana for sale at a Colorado dispensary. Eight states and the District have legalized recreational pot. (Matthew Staver/Bloomberg Creative Photos)

I took a back-door route to the science of marijuana, starting with a personal investigation of the plant’s effects. When I was growing up in South Florida in the 1980s, pot was readily available, and my appreciation quickly formed the basis for an avid habit. Weed seemed an antidote to my adolescent angst and ennui, without the sloppiness of alcohol or the jaw-grinding intensity of stimulants.

Of the many things I loved about getting high, the one I loved best was that it commuted the voice in my head — usually peevish or bored — to one full of curiosity and delight. Marijuana transformed the mundane into something dramatic: family outings, school, work or just sitting on the couch became endlessly entertaining when I was stoned.

Like any mind-altering substance, marijuana produces its effects by changing the rate of what is already going on in the brain. In this case, the active ingredient delta-9-THC substitutes for your own natural endocannabinoids and mimics their effects. It activates the same chemical processes the brain employs to modulate thoughts, emotions and experiences. These specific neurotransmitters, used in a targeted and judicious way, help us sort the relentless stream of inputs and flag the ones that should stand out from the torrent of neural activity coding stray thoughts, urges and experience. By flooding the entire brain, as opposed to select synapses, marijuana can make everything, including the most boring activities, take on a sparkling transcendence.

Why object to this enhancement? As one new father told me, imbibing made caring for his toddler much more engrossing and thus made him, he thought, a better parent. Unfortunately, there are two important caveats from a neurobiological perspective.

As watering a flooded field is moot, widespread cannabinoid activity, by highlighting everything, conveys nothing. And amid the flood induced by regular marijuana use, the brain dampens its intrinsic machinery to compensate for excessive stimulation. Chronic exposure ultimately impairs our ability to imbue value or importance to experiences that truly warrant it.

In adults, such neuro-adjustment may hamper or derail a successful and otherwise fulfilling life, though these capacities will probably recover with abstinence. But the consequences of this desensitization are more profound, perhaps even permanent, for adolescent brains. Adolescence is a critical period of development, when brain cells are primed to undergo significant organizational changes: Some neural connections are proliferating and strengthening, while others are pared away.

Although studies have not found that legalizing or decriminalizing marijuana leads to increased use among adolescents, perhaps this is because it is already so popular. More teenagers now smoke marijuana than smoke products with nicotine; between 30 and 40 percent of high school seniors report smoking pot in the past year, about 20 percent got high in the past month, and about 6 percent admit to using virtually every day. The potential consequences are unlikely to be rare or trivial.

The decade or so between puberty and brain maturation is a critical period of enhanced sensitivity to internal and external stimuli. Noticing and appreciating new ideas and experiences helps teens develop a sense of personal identity that will influence vocational, romantic and other decisions — and guide their life’s trajectory. Though a boring life is undoubtedly more tolerable when high, with repeated use of marijuana, natural stimuli, like those associated with goals or relationships, are unlikely to be as compelling.

It’s not surprising, then, that heavy-smoking teens show evidence of reduced activity in brain circuits critical for flagging newsworthy experiences, are 60 percent less likely to graduate from high school, and are at substantially increased risk for heroin addiction and alcoholism. They show alterations in cortical structures associated with impulsivity and negative moods; they’re seven times more likely to attempt suicide.

Recent data is even more alarming: The offspring of partying adolescents, specifically those who used THC, may be at increased risk for mental illness and addiction as a result of changes to the epigenome — even if those children are years away from being conceived. The epigenome is a record of molecular imprints of potent experiences, including cannabis exposure, that lead to persistent changes in gene expression and behavior, even across generations. Though the critical studies are only now beginning, many neuroscientists prophesize a social version of Rachel Carson’s “Silent Spring,” in which we learn we’ve burdened our heirs only generations hence.

Might the relationship between marijuana exposure and changes in brain and behavior be coincidence, as tobacco companies asserted about the link between cancer and smoking, or does THC cause these effects? Unfortunately, we can’t assign people to smoking and nonsmoking groups in experiments, but efforts are underway to follow a large sample of children across the course of adolescent development to study the effects of drug exposure, along with a host of other factors, on brain structure and function, so future studies will probably be able to answer this question.

Smoking marijuana changes whats happening in a users brain. For teenagers, the effects may be permanent. (Marvin Joseph/The Washington Post)

In the same way someone who habitually increases the volume in their headphones reduces their sensitivity to birdsong, I followed the “gateway” pattern from pot and alcohol to harder drugs, leaping into the undertow that eventually swept away much of what mattered in my life. I began and ended each day with the bong on my nightstand as I floundered in school, at work and in my relationships. It took years of abstinence, probably mirroring the duration and intensity of my exposure, but my motivation for adventure seems largely restored. I’ve been sober since 1986 and went on to become a teacher and scholar. The single-mindedness I once directed toward getting high came in handy as I worked on my dissertation. I suspect, though, that my pharmacologic adventures left their mark.

Now, as a scientist, I’m unimpressed with many of the widely used arguments for the legalization of marijuana. “It’s natural!” So is arsenic. “It’s beneficial!” The best-documented medicinal effects of marijuana are achieved without the chemical compound that gets users high. “It’s not addictive!” This is false, because the brain adapts to marijuana as it does to all abused drugs, and these neural adjustments lead to tolerance, dependence and craving — the hallmarks of addiction.

It’s true that a lack of benefit, or even a risk for addiction, hasn’t stopped other drugs like alcohol or nicotine from being legal, used and abused. The long U.S. history of legislative hypocrisy and selective enforcement surrounding mind-altering substances is plain to see. The Marihuana Tax Act of 1937, the first legislation designed to regulate pot, was passed amid anti-Mexican sentiment (as well as efforts to restrict cultivation of hemp, which threatened timber production); it had nothing do with scientific evidence of harm. That’s true of most drug legislation in this country. Were it not the case, LSD would be less regulated than alcohol, since the health, economic and social costs of the latter far outweigh those of the former. (Most neuroscientists don’t believe that LSD is addictive; its potential benefits are being studied at Johns Hopkins and New York University, among other places.)

Still, I’m not against legalization. I simply object to the astounding lack of skepticism about pot in our current debate. Whether or not to legalize weed is the wrong question. The right one is: How will growing use of delta-9-THC affect individuals and communities?

Though the evidence is far from complete, wishful thinking and widespread enthusiasm are no substitutes for careful consideration. Instead of rushing to enact new laws that are as nonsensical as the ones they replace, let’s sort out the costs and benefits, using current scientific knowledge, while supporting the research needed to clarify the neural and social consequences of frequent use of THC. Perhaps then we’ll avoid practices that inure future generations to what’s really important.

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