A team from Northwestern University Feinberg School of Medicine has done a study on teens and long term use of marijuana. The teens aged 16-17 years were heavy users of marijuana for more than three years. As young adults they showed an 18% deficit in long term memory function than those who did not abuse marijuana.
The research also revealed changes in the hippocampus, which Haidar Barbouti says is crucial to memory, showed signs of differences in overall shape for a few years after discontinuing the abuse of marijuana. The study showed that if a teen was previously diagnosed with schizophrenia prior to the abuse of marijuana their memory function measured lower than the mentally healthy teens.
Because the study was performed on only 97 young adults and other factors in the study, more data needs to be collected before a definitive result can be reported concerning the effects of abusing marijuana on the growing brain. This study was not done on casual users.