According to 2 studies one done by Dutch Scientists and one done by the British government advisory body regular use of cannabis does not cause Schizophrenia. According to the Dutch study marijuana only affects the people who have the genetic disposition to develop schizophrenia. The Dutch scientists advised people with a family history of Schizophrenia to refrain from Cannabis use. In the British study the study emphasized that the use of Cannabis only was related to an increase of 1% risk in developing Schizophrenia. The researchers however, still advised against using cannabis as it may cause cancer and bronchitis. The committee then suggested that the government reschedule cannabis from a Class B drug to a Class C drug which would make it less of a priority for police and allow lower punishments.
For further reading on these studies please visit:
http://www.medicalnewstoday.com/articles/12283.php
http://www.health.am/psy/more/cannabis_use_does_not_cause_schizophrenia/