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Marijuana May Hurt The Developing Teen Brain

Above: The teenage years are the last golden opportunity to build a healthy brain, researchers say. So smoking pot might not be so smart.

The brain of a teenager has a lot of developing to do. It must transform itself from the brain of a child into the brain of an adult. And some researchers worry how marijuana might affect that crucial process.

"Actually in childhood our brain is larger," says Krista Lisdahl, director of the University of Wisconsin-Milwaukee brain imaging and neuropsychology "Then during the teenage years our brain is getting rid of those connections that weren't really used and it prunes back. It actually makes the brain faster and more efficient."

The streamlining process ultimately helps the brain make judgments, think critically and remember what it's learned.

Lisdahl says it's a mistake for teenagers to use cannabis. "It's the absolute worst time," she says, because the mind-altering drug can disrupt development. Think of the teen years, she says, as the "last golden opportunity to make the brain as healthy and smart as possible."

Lisdahl points to a growing number of studies that show regular marijuana use – once a week or more — actually changes the structure of the teenage brain, specifically in areas dealing with memory and problem solving.

That can affect cognition and academic performance, she says. "And, indeed, we see if we look at actual grades that chronic marijuana using teens do have, on average, one grade point lower than their matched peers that don't smoke pot," Lisdahl says.

In one study, researchers from Duke University analyzed data gathered over many years from people living in New Zealand. They compared IQs in childhood through age 38 among marijuana users and non-users.

"We found that people who began using marijuana in their teenage years and then continued to use marijuana for many years lost about 8 IQ points from childhood to adulthood," says study author Madeline Meier, now a professor at Arizona State University, "whereas those who never used marijuana did not lose any IQ points."

And the amount people smoked made a difference. Those who smoked the most — at least every day – saw the greatest drop in IQ, the full 8 points. And the younger they were when they started using cannabis, the greater the IQ decline.

And it wasn't just IQ. Adults who smoked marijuana as teenagers did worse in tests of memory and decision making than adults who hadn't smoked pot.

But there's an important caveat here. Those who used the most marijuana in the Meier study had lower IQs to begin with. Dr. Gregory Tau, a psychiatrist and drug abuse researcher at Columbia University, says there's a chicken and egg dilemma with many marijuana studies, including this one.

"It's very possible that there's something very different to begin with among teenagers who tend to get into trouble with marijuana or who become heavy users," Tau says. "They could have subtle emotional differences, perhaps some cognitive functioning differences. It may be hard for them to 'fit in' with a peer group that's more achievement oriented

These differences could predispose them to use pot.

Tau says more funding is needed for better-designed long-term studies. Current research is inadequate to answer all the questions about marijuana risk.

But he says some things are common sense. "It's not rocket science to think if you smoke weed when your brain is developing, that it can't be 'good' for you, just like any 'toxic' substance isn't good for you," he says.

Such concern seems to be lost among teenagers themselves. In a recent federally sponsored survey, 60 percent of high school seniors say they think marijuana is safe. And 23 percent say they've used marijuana in the last month – more than those who used alcohol or smoked cigarettes.

Six percent of high school seniors say they use pot every day, which is triple the rate over the past decade.

And, the marijuana they smoke is a lot more potent than it was in the 1970s, with far higher levels of THC, the main mind altering ingredient. "The higher the THC levels, the more brain changes there are and the more there is the risk for addiction," Krista Lisdahl says.

Lisdahl says more teens and young adults are smoking marijuana in states that have made the drug available for medical use. She says that's worrisome because it might be a harbinger of things to come if pot is fully legalized.

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Avatar for user 'Dennis Russell'

Dennis Russell | March 3, 2014 at 7:34 a.m. ― 3 years ago

It looks like marijuana can be added to the expanding list of catalysts to the dumbing down of the USA.

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Avatar for user 'casualobserver'

casualobserver | March 3, 2014 at 10:12 a.m. ― 3 years ago

It won't matter what marijuana does to the brain because the only factor that matters in the good old USA is how much money it can generate. We already know alcohol and tobacco cause physical damage but we sure do like those tax revenues! Colorado just made $134 million in tax revenues from legalizing marijuana and being the whores we are in this country we'll patriotically justify it's availability as a personal choice issue because we just aren't going to let that kind of money pass us by. Of course we"ll offer the usual meaningless assertions that it's for adults only and it should be used responsibly so we can wash out hands of any responsibility for its impact.

For what it's worth I don't care either way. Legal, illegal whatever; let's just be honest about our motives. And what I want to know is why, if life is so wonderful in this country, do so many people need drugs?

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Avatar for user 'JeanMarc'

JeanMarc | March 3, 2014 at 10:21 a.m. ― 3 years ago

This is easy to see. Look at any pot head kid who smoked weed every day while in high school. They are all dopey idiots. It's ok, more of them means less competition for people like me. I will always need some loser to cook my fries.

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Avatar for user 'xwagner'

xwagner | March 3, 2014 at 12:17 p.m. ― 3 years ago

This story is torn from the headlines of 1970. It was ridiculed on the NPR website.

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Avatar for user 'JeanMarc'

JeanMarc | March 3, 2014 at 3:29 p.m. ― 3 years ago

xwagner are you really saying you see no connection between smoking pot everyday and acting like a typical brain dead stoner? It is just a coincidence that all people who smoke pot act retarded? Perhaps you think the only people who smoke pot were retarded to begin with, and that is why they act that way. Surely it is not from the marijuana, right?

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Avatar for user 'Right_sure'

Right_sure | March 3, 2014 at 4:42 p.m. ― 3 years ago

Wwagner smokes a lot when he was a teenager apparently.

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Avatar for user 'Boots'

Boots | March 3, 2014 at 7:39 p.m. ― 3 years ago

What a dumb story, you could just as well substitute cigarettes,alcohol,cheezeburgers
etc. in the headline.

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Avatar for user 'Boots'

Boots | March 3, 2014 at 7:43 p.m. ― 3 years ago

Thank god teenagers don't watch fox snooze

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Avatar for user 'xwagner'

xwagner | March 4, 2014 at 6:33 a.m. ― 3 years ago

Ah, ad hominem attacks from the backwater population. Enjoy the ride, kids. Rub both brain cells together for warmth if need be.

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Avatar for user 'JeanMarc'

JeanMarc | March 4, 2014 at 8:30 a.m. ― 3 years ago

I see the two cat lovers have the same opinion as each other, and most other leftists.

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Avatar for user 'DeLaRick'

DeLaRick | March 4, 2014 at 9:36 a.m. ― 3 years ago

The negative effects of our idiotic popular culture and media are more detrimental to developing minds than any drug or substance.

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Avatar for user 'JeanMarc'

JeanMarc | March 4, 2014 at 11:08 a.m. ― 3 years ago

DeLaRick why don't you tell that to a meth addict, crack addict, or alcoholic. I assure you they are damaged far beyond what pop culture could do.

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Avatar for user 'DeLaRick'

DeLaRick | March 4, 2014 at 12:09 p.m. ― 3 years ago


Our culture is a gateway to drugs and all kinds of self-abuse. People don't know how to let themselves, and others, just be. Do you think our culture doesn't have anything to do with our total saturation of illicit and prescription drugs? I'm not absolving individuals for their poor choices, but we're not a nation of junkies by random accident.

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Avatar for user 'JeanMarc'

JeanMarc | March 4, 2014 at 2:37 p.m. ― 3 years ago

DeLaRick what you say is true, this culture propelled forward by the leftist media is a big factor in the poor choices of our youth today. The leftists know that they can hinder children from becoming self-reliant adults if they get them interested in drugs and alcohol, and disinterested in a college degree.

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Avatar for user 'Peking_Duck_SD'

Peking_Duck_SD | March 4, 2014 at 3:50 p.m. ― 3 years ago

JeanMarc: "The leftists know that they can hinder children from becoming self-reliant adults if they get them interested in drugs and alcohol, and disinterested in a college degree."


I thought it was the right wing crazies who hate colleges - all those scary liberal professors and all.

And the right wing nutjobs on Fox "news" are always talking about colleges and universities being places of "left-wing indoctrination".

You get the loser "Joe the Plumber" types buying into the right-wing fantasy that people are better off if they don't go to college and instead "make it" on their own.

Sean Hannity.

Rush Limbaugh.

Glenn Beck.

What do the 3 of them have in common?

They all failed out of college.

They didn't simply decide they didn't want to go, no they tried and literally FAILED right out because they were too butt-stupid to get through it.

I think it's pretty rich that you, Jean, are implying "leftists" want to PREVENT people from going to college when all the squawking right-wing dopes are saying "leftists" WANT young men and women to go to college so they can be "indoctrinates".

So which is it?

By the way, it's President Obama and Democrats who are in favor of trying to EXPAND college enrollment through a variety of programs, but Republicans are against spending any money to increase college enrollment.

So you comment seems "off".

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Avatar for user 'Peking_Duck_SD'

Peking_Duck_SD | March 4, 2014 at 3:55 p.m. ― 3 years ago

And as far as how MJ plays into all this - I've seen high school pot heads who are now failures as well as HS potheads who are not very successful.

I've seen people who never touched MJ in their lives who are failures, and some who are successful.

It's a pretty far-fetched extrapolation to take the data from this one study and imply it's proof that people who smoke weed don't do as well in life.

And it's really irrelevant to the legalization effort because if legalized it would be for people either 18+ or 21+ much like alcohol and tobacco.

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Avatar for user 'Peking_Duck_SD'

Peking_Duck_SD | March 4, 2014 at 3:57 p.m. ― 3 years ago

Correction to my first sentence above, I have seen pot heads in HS who ARE very successful today, as well as others who aren't. It's not the weed that determines this my dears.

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