Marijuana and the Developing Brain: Current State of Evidence

Hansa Bhargava, MD


April 20, 2018

Hello. I'm Dr Hansa Bhargava, a practicing pediatrician and a medical editor for Medscape and WebMD.

The push to legalize recreational marijuana continues to grow even while research on effects of long-term use is inconclusive. At present, 29 states allow medical use of marijuana with a physician's prescription. There are nine states where pot is considered legal and sales for recreational purposes are allowed. Another 13 states have decriminalized its use.

Should marijuana be allowed for recreational use? Do we know enough about its long-term effects, especially on teens' and children's brains?

There are some legitimate concerns.

We know that cannibidol (CBD), the nonpsychoactive component of cannabis, has been shown to be beneficial in treating pain, seizure disorders, and nausea from cancer chemotherapy.

But new studies indicate that the psychoactive component of pot—tetrahydrocannabinol (THC)—could have some risks associated with long-term use, especially in adolescents whose brains are still developing.[1]

A New Zealand study[2] followed a group of people born in 1972 who began smoking pot as teens. Persistent marijuana users—those who used consistently from the time of study entry at 18 years of age until conclusion at 38 years—experienced a drop in IQ of as much as six points. This reduction is similar to that associated with lead exposure.

And then there is the question of the [detrimental] mental health effects of frequent marijuana use.

A review study[3] published in March 2017 concluded that a connection between the two could exist, especially in adolescents. More research, however, is needed.

For those who carry the AKT1 gene, the risk could be particularly high. New research out of the University of Exeter[4] shows that genetics could play a role in vulnerability to the effects of marijuana. Daily use in persons with an AKT1 polymorphism carries double the risk for cannabis-induced psychosis.

Another concern is in regard to the hypothesis that marijuana use fosters apathy—the so-called marijuana amotivational syndrome. A study[5] of 500 college students published earlier this year lent some support to this hypothesis. On the basis of self-reported use and measures of self-efficacy, the researchers concluded that marijuana use was associated with lower initiative and persistence, underscoring marijuana as a risk factor for decreased general self-efficacy. Yet, none of these studies is entirely conclusive, and our knowledge about long-term effects of marijuana on developing brains is incomplete.

In a review[6] of more than 10,000 studies of marijuana use, researchers from the National Academies of Science, Engineering, and Medicine found little definitive evidence, positive or negative, regarding long-term use of pot.

As such, the word "caution" continues to be used in this arena, especially in regard to marijuana use by children and teens, whose brains are still developing and could be more susceptible.

For Medscape and WebMD, I'm Dr Hansa Bhargava.

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