This was published in the peer reviewed "Current Psychiatry" 2/07. I cannot post the article in full b/c of publishing issues, but here is a summary:
"Mj can cause psychotic illness, particularily in persons at risk. Treat both the psychosis and substance abuse. Psychoeducation might be as effective as cannabis-specific treatments. Patients with a psychotic illness who use mj have a worse prognosis than nonusers."
The persons at risk are adolescents whose brains are still developing and have a genetic predisposition to psychotic illnesses, such as the schizophrenic spectrum of disorders.
Patients who have psychotic illnesses and use mj, compared to nonusers, tend to have earlier onset of schizophrenia, more psychotic symptoms, worse prognosis b/c of poorer treatment adherence, increased symptom severity and persistence, and higher relaspse rate.