Teen cannabis use linked to poor memory, study shows

Regular, long-term marijuana use by teenagers may damage brain structures related to memory, researchers say

In an April 20, 2005 file photo, a University of Colorado freshman, who did not want to be identified, joins a crowd smoking marijuana during a '420' gathering.
Richard M Hackett/AP

Teens who smoked marijuana on a daily basis had abnormal brain structure changes, damaging their memory, after long-term use, a Northwestern University Medicine study found.

The younger the individuals were when they started regularly using marijuana, the more abnormalities were found in their brains – suggesting the brain may be more susceptible to the effects of a drug if the habit starts at an early age.

The individuals observed in the study began using marijuana at ages 16 and 17 and smoked every day for about three years.

It is the first study to target regions deep in the brain related to motivation and memory with MRI technology, and to correlate abnormalities in those regions with an impaired working memory.

Working memory is the ability to remember and process information in the moment and, if needed, transfer it to long-term memory.

After chronic, prolonged cannabis use, the structures related to working memory in the teens' brains appeared to shrink and cave in. Some scientists believe changes in brain structure can change the way the brain functions.

The brain abnormalities and memory problems were observed after the teens had stopped smoking marijuana for two years – indicating long-term effects of regular use.

“The study links the chronic use of marijuana to these concerning brain abnormalities that appear to last for at least a few years after people stop using it,” said lead study author Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine.

“With the movement to decriminalize marijuana, we need more research to understand its effect on the brain.”

Effects of legalization

Marijuana is the most commonly used illegal drug in the U.S., and some believe decriminalization could lead to greater use.

But others say prohibiting cannabis may be more dangerous than regulating the substance — including those who advocated for the campaign to legalize marijuana for use over the age of 21 in the states of Colorado and Washington, which was approved by voters in both states in November.

Mason Tvert, co-director of the Campaign to Regulate Marijuana Like Alcohol, which is the main group behind the legalization measure in Colorado, told Reuters that voters see prohibition as the "worst possible policy" for protecting teenagers.

"Because it's putting marijuana in an underground market, where it's entirely uncontrolled," he said.

His group pointed to a survey by the federal Centers for Disease Control and Prevention which showed that while pot use increased nationwide, the percentage of teen users in Colorado fell to 22 percent in 2011 from 25 percent in 2009.

That coincided with the creation of a state agency in 2010 to oversee medical marijuana use, which Tvert's group says shows that regulating pot — not prohibiting it — may reduce teen use.

An October Gallup Poll showed that for the first time, most Americans are in favor of legalizing marijuana — 58 percent said it should be legal for both recreational and medical use.

Alcohol, an even more popular intoxicant among teens, has been shown to have detrimental effects on the brain.

Teenagers who consumed alcohol are likely to have reduced white matter brain tissue health, while those who consumed marijuana are not, according to a study conductd by researchers at the University of California San Diego, published in the April 2012 issue of the journal Alcoholism: Clinical & Experimental Research.

Reduced brain tissue could mean declines in memory, attention, and decision-making into later adolescence and adulthood, the researchers said.

Al Jazeera and wire services

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