Teenagers who use marijuana may cause permanent damage to their brains, according to a study from the University of Maryland School of Medicine.

"Children who start [using marijuana] around pre-adolescence, 13 to 15 years of age, tend to develop very severe deficits," says Dr. Asaf Keller, a professor of neurobiology at the U.S. university.

According to Keller, the effects include a high incidence of schizophrenia and attention deficit disorder as well as "long term and permanent reductions in intelligence measured by IQ tests."

As part of the study, published in the online edition of the journal Neuropsychopharmacology in July, researchers tested the brain activity of young mice that had been exposed to low levels of the active ingredient in marijuana for 20 days.

When the mice were tested in adulthood, the part of the brain responsible for cognitive functions — including sensory processing and motor skills — were completely abnormal.

Keller said the teenage brain reacts differently to marijuana than the adult brain because the frontal part of the brain is still developing during adolescence.

The fact that the brain showed irreversible changes is worrisome, he added.

"Even complete cessation of exposure will not reverse these changes," he said.

Even more worrisome is that many young people don't seem to understand the dangers of using marijuana. A study published last month by the Canadian Centre on Substance Abuse examined youth attitudes towards marijuana.

"Many of them perceived it as a natural and harmless substance," says Amy Porath-Waller, a senior researcher and policy analyst for the CCSA.

"Many young people thought it increases their concentration and focus to help them at school. They think it makes them a better driver on the road. Some youth even thought that cannabis can cure cancer."

The CCSA study also showed that teens think everyone is smoking weed all of the time.

"There's this normalization of the behaviour," says Porath-Waller.

That normalization may encourage more kids to try marijuana in the first place. The 2012 "Monitoring the Future" report shows as the perceived risk of using marijuana goes down among teenagers, pot use goes up.