Heavy Marijuana Use Shrinks Your Brain But Increases Connectivity

There’s a reason why every new study on marijuana sparks heated debate—even scientists can’t seem to agree on its long-term effects.

One study suggests it lowers sperm quality, while another claims it could be a breakthrough treatment for autoimmune diseases.

Some researchers argue it has no impact on IQ, while others insist that teenage users experience cognitive decline.

The contradictions are endless.

But now, for the first time, neuroscientists at the University of Texas have conducted a comprehensive investigation into the long-term effects of marijuana use—and their findings may finally explain why the science has been so unclear.

The results show that marijuana users’ brains are fundamentally different from those who don’t use the drug.

And the changes aren’t uniform. Instead, they vary based on how long someone has used marijuana and at what age they started.

This groundbreaking study sheds light on how marijuana affects brain volume, connectivity, and even IQ—sometimes in unexpected ways.

The findings could reshape the conversation around marijuana’s risks and benefits, especially as legalization spreads across the world.

What Happens to the Brain After Years of Marijuana Use?

Researchers at the Centre for Brain Health at the University of Texas recruited 48 chronic marijuana users and compared them to 62 non-users matched by age and gender.

These weren’t occasional users—each participant had used marijuana an average of three times a day for 10 years.

Importantly, none had a history of psychosis or neurological disorders.

The team performed MRI scans to measure three key brain characteristics:

The volume of the orbitofrontal cortex (OFC), the brain region linked to decision-making and reward processing.
The connectivity between the OFC and other brain regions.
The structural integrity of brain tissue.

The results?

Chronic marijuana users had a smaller OFC than non-users.

But here’s where things get interesting: Their brains were also compensating for this shrinkage.

MRI scans showed that marijuana users developed stronger connectivity between different brain regions, as if the brain were rewiring itself to offset the loss.

“The orbitofrontal cortex is one of the primary regions in a network of brain areas called the reward system,” lead researcher Francesca Filbey explained.

“It helps us determine what is good for us and what keeps us sustained.”

This suggests that prolonged marijuana use directly affects the brain’s reward system.

The Brain Adapts—Until It Doesn’t

At first, this increased connectivity might seem like a good thing.

The brain appears to adapt to the changes, possibly minimizing any negative cognitive effects.

But here’s the catch: After six to eight years of chronic marijuana use, this increased connectivity starts to decline.

Over time, long-term users end up with both a shrunken OFC and only slightly above-average brain connectivity.

Even more surprising, the study found that IQ scores were lower in marijuana users—but not necessarily because of the reduced OFC volume.

This suggests that the connection between marijuana use and cognitive decline is more complex than previously thought.

Does Marijuana Permanently Lower IQ?

For years, the claim that marijuana lowers IQ has been a go-to argument against legalization.

This study challenges that idea in an unexpected way.

Yes, chronic users had lower IQ scores.

But researchers found no direct link between IQ and the size of the orbitofrontal cortex.

In other words, marijuana-related brain changes may not be the sole cause of IQ decline.

This finding aligns with previous research from 2012, which suggested that people with naturally smaller OFCs at age 12 were more likely to start using marijuana later in life.

This raises an important question: Does marijuana shrink the OFC, or are people with smaller OFCs more likely to use marijuana?

The answer remains unclear.