Does Cannabis cause Schizophrenia?


When in 1954 Doll and Hill1 published their landmark paper that linked cigarette smoking with lung cancer, they probably did not consider it would take the medical zeitgeist another decade to acknowledge the association as causal formally, evident in the Surgeon General's 1964 report2. The tobacco industry’s Master Settlement Agreement on the link came over three decades later3. It is therefore reasonable to expect that the debate over the link between cannabis and schizophrenia would likely linger on for some time to come.


Indeed, that the debate over the legal status of cannabis invariably creeps into the picture obscures the more important public health issues concerning cannabis. These include, besides educational and vocational breakdown, and increased predisposition to criminal behavior with probable even significant adverse health consequences, its other potential health hazards such as cognitive dysfunction. It also obscures the need for youths to avoid its use, known to result  in the use of tobacco, whose toxins it could enhance, hence the risk of developing lung cancer, not to mention making it difficult to quit smoking. It even increases the risk of using other drugs such alcohol, amphetamines, and cocaine.


The nature of the established link between cannabis use and psychosis, not to mention schizophrenia, remains contentious4. Does cannabis precipitate psychosis in prone persons5, which might explain why there has not been an explosion of psychoses in youths considering the increased rates of cannabis use among them recently, or does it serve to medicate psychosis6, among some explanations given for this link? Whatever the answer, that there is a link between cannabis and psychosis is not in question.


Thus, we need to establish appropriate interventions to prevent cannabis use among all youths, even if we only went by current somewhat inconclusive evidence of the causal role of cannabis in psychosis and indeed, schizophrenia. Some researchers claim that almost half of schizophrenia according to some estimates is genuinely attributable to cannabis use, and that cannabis could increase overall lifetime risk of developing this disease from 0.7 to 1.4 in 10006, the risk even higher in the vulnerable. Other investigators, able to control for psychotic symptoms occurring before cannabis use found an increase in ‘schizophrenia symptoms’ but not schizophrenia7.


Thus, could we prevent schizophrenia? Even if we could not, we could, the variety of harm cannabis causes, significantly. Our young people need to know the mental health and other risks of using cannabis, and we should in particular discourage young persons with mental health issues from using cannabis, as this might set off a major psychotic episode. Should we then not formulate the appropriate public policy in response to these issues?






1. Doll R, Hill AB. The mortality of doctors in relation to their smoking habits: a preliminary reportBMJ 1954; 4877: 1451-1455.

2. Surgeon-General. Smoking and health: report of the advisory committee to the surgeon general of the public health serviceWashington DC, USADepartment of Health, Education, and Welfare, 1964:

3. King C 3rd, Siegel M. The master settlement agreement with the tobacco industry and cigarette advertising in magazinesN Engl J Med 2001; 345: 504-511.

4. Hall WD. Cannabis and psychosisDrug Alcohol Rev 1998; 17: 433-444.

5. Zammit S, Allebeck P, Andreasson S, Lundberg I, Lewis G. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort studyBMJ 2002; 325: 1199-1201.

6. Saha S, Chant D, Welham J, McGrath J. A systematic review of the prevalence of schizophreniaPLoS Med 2005; 2: e14118. 

7. Arseneault L, Cannon M, Poulton R, Murray R, Caspi A, Mofitt T. Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. BMJ 2002; 325:1212-3