Toronto: Individuals who were exposed to marijuana from a young age can have highly abnormal brain functioning and are also likely to be low on intelligence quotient (IQ) levels, a study has found.

Marijuana is the most commonly used illegal substance in the world. 

The findings showed that participants who started using marijuana from early in life had highly abnormal brain function in areas related to visuo-spatial processing, memory, self-referential activity and reward processing. 

The study found no evidence that marijuana use improved depressive symptoms, there was no difference in psychiatric symptoms between those with depression who used marijuana and those with depression who did not use marijuana, the researchers said.

The use of marijuana did not correct the brain function deficits of depression, and in some regions made them worse.

"The study suggest that using marijuana does not correct the brain abnormalities or symptoms of depression and using it from an early age may have an abnormal effect not only on brain function, but also on IQ," said Elizabeth Osuch from Lawson Health Research Institute, in Ontario, Canada. 

Previous studies have suggested that frequent marijuana users, especially those who begin at a young age, are at a higher risk for cognitive dysfunction and psychiatric illness, including depression, bipolar disorder and schizophrenia.

For the study, Osuch and her team recruited youth in four groups: those with depression who were not marijuana users, those with depression who were frequent marijuana users, frequent marijuana users without depression, and healthy individuals who were not marijuana users. 

Participants were later divided into youth who started using marijuana before the age of 17 and those who began using it later or not at all. They then underwent psychiatric, cognitive and IQ testing as well as brain scanning. 

The results, published in the journal Acta Psychiatrica Scandinavica, showed differences in brain function among the four groups in areas of the brain that relate to reward-processing and motor control.