"For the first time, researchers at Northwestern University have analyzed the relationship between casual use of marijuana and brain changes – and found that young adults who used cannabis just once or twice a week showed significant abnormalities in two important brain structures.

The study’s findings, to be published Wednesday in the Journal of Neuroscience, are similar to
those of past research linking chronic, long-term marijuana use with mental illness and changes in brain development.  

Dr. Hans Breiter, co-senior study author, said he was inspired to look at the effects of casual marijuana use after previous work in his lab found that heavy cannabis use caused similar brain abnormalities to those seen in patients with schizophrenia."