Early use of marijuana – the most commonly used illegal substance in the world – may cause abnormal brain function and lower IQ, a new study has warned.
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.
“Many youth in our program use marijuana heavily and, despite past research, believe it improves their psychiatric conditions because it makes them feel better momentarily,” said Elizabeth Osuch, a scientist at Lawson Health Research Institute and Western University in Canada.
“For this reason, we decided to study the effects of marijuana and depression on psychiatric symptoms, brain function and cognitive function,” said Osuch.
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.
In addition, 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.
Participants underwent psychiatric, cognitive and IQ testing as well as brain scanning.
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.
In addition, results showed differences in brain function among the four groups in areas of the brain that relate to reward-processing and motor control.
The use of marijuana did not correct the brain function deficits of depression, and in some regions made them worse.
Those participants who used marijuana from a young age had highly abnormal brain function in areas related to visuo-spatial processing, memory, self-referential activity and reward processing.
The study found that early marijuana use was also associated with lower IQ scores.
“These findings 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 Osuch.