New research may give backing to parents telling teens to “just say no.” A study in mice from the University of Maryland School of Medicine reveals that regular use of marijuana during adolescence could damage brain function, potentially increasing the risk for schizophrenia and other psychiatric problems.

The study, which was recently published in Neuropsychopharmacology, had scientists examining cortical oscillations – patterns of the brain’s neuron activity – in mice. These oscillations become abnormal when schizophrenia or other psychiatric disorders are present.

Researchers exposed young mice to low doses of tetrahydrocannabinol (THC), which is the active ingredient present in marijuana, for 20 days. Then the mice were returned to their family to continue developing.

When the mice became adults, the scientists discovered that their cortical oscillations were severely modified, and the mice showed signs of impaired cognitive functions.

Sylvina Mullins Raver from the University of Maryland says: “The striking finding is that, even though the mice were exposed to very low drug doses, and only for a brief period during adolescence, their brain abnormalities persisted into adulthood.”

Interestingly, the researchers originally set out to test a hypothesis that marijuana use effects cortical oscillations in adults. But when they repeated the experiment on adult mice who had never been exposed to the drug previously, they found that both the oscillations and cognitive behavior stayed normal.

So, it appears that adolescence is the period of development when marijuana use can take its biggest toll. Senior author Dr. Asaf Keller adds:

We looked at the different regions of the brain. The back of the brain develops first, and the frontal parts of the brain develop during adolescence.

We found that the frontal cortex is much more affected by the drugs during adolescence. This is the area of the brain that controls executive functions, such as planning and impulse control. It is also the area most affected in schizophrenia.”

Now that marijuana has been legalized in 19 US states and the District of Columbia, the study’s results about long-term effects of marijuana use are particularly relevant. The authors point to controversy spanning back to 20 years ago about whether or not marijuana use can cause permanent damage and contribute to psychiatric disorders.

Dr. Keller adds: “There is likely a genetic susceptibility, and then you add marijuana during adolescence and it becomes the trigger.”

The research team will continue to study the changes in cortical oscillations with a view to potentially reverse the effects some day.

“We are hoping we will learn more about schizophrenia and other psychiatric disorders, which are complicated conditions,” Dr. Keller says.