Joseph M. Rey, MBBS, PhD, Honorary Professor in the Department of Psychological Medicine at the University of Sydney, and Christopher C. Tennant, MD, Professor of Psychiatry at the University of Sydney, wrote in a Nov. 23, 2002 editorial review in the British Medical Journal:
"Establishing direction of causality is difficult and is most appropriately assessed in non-clinical samples, but a low incidence of the illness and the fact the most drug users take other drugs in addition to cannabis create methodological problems and explain the dearth of reliable evidence."
Joseph M. Pierre, MD, Co-Chief of the Schizophrenia Treatment Unit at the Veterans Administration (VA) West Los Angeles Healthcare Center, wrote in his Sep. 2011 article "Cannabis, Synthetic Cannabinoids, and Psychosis Risk: What the Evidence Says" in Current Psychiatry:
"There are several ways to explain the link between cannabis use and psychosis, and a causal relationship has not yet been firmly established. Current evidence supports that cannabis is a 'component cause' of chronic psychosis, meaning although neither necessary nor sufficient, cannabis use at a young age increases the likelihood of developing schizophrenia or other psychotic disorders. The overall magnitude of risk appears to be modest, and cannabis use is only 1 of myriad factors that increase the risk of psychosis. Furthermore, most cannabis users do not develop psychosis. However, the risk associated with cannabis occurs during a vulnerable time of development and is modifiable."
[Editor's Note: Dr. Pierre's table of hypotheses linking cannabis, psychosis, and strength of evidence may be found here (493 KB) ]
Cécile Henquet, PhD, Researcher in the Department of Psychiatry and Neuropsychology at Maastricht University, et al., wrote the following in their June 2010 article titled "Psychosis Reactivity to Cannabis Use in Daily Life: An Experience Sampling Study," published in the British Journal of Psychiatry:
"Cannabis use in daily life was associated with subsequent increases in hallucinatory experiences, in particular auditory hallucinations. Patients with a psychotic disorder [such as schizophrenia or schizoaffective disorder] were more sensitive to the hallucinogenic effects of cannabis than healthy controls... The data suggest that the positive effects of cannabis on mood are acute [occurring in the short term] whereas its association with psychotic experiences is sub-acute [occurring in the longer term]."
[Editor's Note: Dr. Henquet’s June 2010 study above is classified as Not Clearly Pro or Con because our question is whether or not marijuana causes psychotic disorders whereas her study – done exclusively on marijuana smokers with existing psychotic disorders – suggests the subjects experience heightened short-term positive effects and long-term negative effects from marijuana use.
Prior to her June 2010 statement above, Dr. Henquet published a Dec. 2004 article titled "Prospective Cohort Study of Cannabis Use, Predisposition for Psychosis, and Psychotic Symptoms in Young People," published in the British Medical Journal, with a statement that is directly Pro to our question "Does marijuana use cause lasting schizophrenia, psychosis, or other mental disorders?," as seen in the Pro column below.]
Does Marijuana Use Cause Lasting Schizophrenia, Psychosis, or Other Mental Disorders?
Tauheed Zaman, MD, Clinical Fellow in Psychiatry at the Cambridge Health Alliance, et al., stated the following in their Oct. 2013 "Resource Document on Marijuana as Medicine" for the American Psychiatric Association, available at psych.org:
"Several studies have shown that cannabis use may in fact exacerbate or hasten the onset of psychiatric illnesses... This includes the contribution of marijuana to symptoms of mood disorders, anxiety and psychosis, particularly in young adulthood. Cannabis use is associated with the emergence of mood disorders, particularly symptoms of bipolar disorder, among those with a family history of mood disorder. Among those with a predisposition to psychotic disorders, cannabis may hasten the emergence of both positive and negative psychotic symptoms...
Cannabis use is associated with increased depression, suicidal ideation, use of other substances and risky behavior among adolescents. Regular adolescent cannabis use is also associated with increased incidence of anxiety disorders. Cannabis use significantly increases the risk of psychotic disorders among young adults... These findings are of particular concern as symptoms often persist into adulthood, and therefore cannabis use may increase the risk of lifelong symptoms and disability due to mental illness."
Theresa H. M. Moore, MSc, Research Associate in the Department of Social Medicine at the University of Bristol, and Stanley Zammit, PhD, MB, MA, Clinical Lecturer in the Department of Psychological Medicine at Cardiff University, wrote the following in their July 28, 2007 article titled "Cannabis Use and Risk of Psychotic or Affective Mental Health Outcomes: A Systematic Review," published in The Lancet:
"There was an increased risk of any psychotic outcome in individuals who had ever used cannabis…with greater risk in people who used cannabis most frequently. There is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life."
John McGrath, MBBS, MD, PhD, Psychiatrist at the Queensland Centre for Mental Health Research, et al., stated the following in their May 2010 article titled "Association Between Cannabis Use and Psychosis-Related Outcomes Using Sibling Pair Analysis in a Cohort of Young Adults," published in Archives of General Psychiatry:
"Longer duration since first cannabis use was associated with multiple psychosis-related outcomes in young adults... the longer the duration since first cannabis use, the higher the risk of psychosis-related outcomes...
Compared with those who had never used cannabis, young adults who had 6 or more years since first use of cannabis (i.e., who commenced use when around 15 years or younger) were twice as likely to develop a nonaffective psychosis...
This study provides further support for the hypothesis that early cannabis use is a risk-modifying factor for psychosis-related outcomes in young adults."
Cécile Henquet, PhD, Researcher in the Department of Psychiatry and Neuropsychology at Maastricht University, et al., in their Dec. 2004 article "Prospective Cohort Study of Cannabis Use, Predisposition for Psychosis, and Psychotic Symptoms in Young People," published in the British Medical Journal, stated:
"Exposure to cannabis during adolescence and young adulthood increases the risk of psychotic symptoms later in life. Cannabis use at baseline increased the cumulative incidence of psychotic symptoms at follow up four years later...but has a much stronger effect in those with evidence of predisposition for psychosis."
Stanley Zammit, PhD, MB, MA, Clinical Lecturer in Psychiatric Epidemiology at Cardiff University, and Peter Allebeck, MD, Professor in Social Medicine at the Karolinska Institutet, in a study of over 50,000 Swedish conscripts published Nov. 23, 2002 in the British Medical Journal wrote:
"Cannabis use is associated with an increased risk of developing schizophrenia, consistent with a causal relation. This association is not explained by use of other psychoactive drugs or personality traits relating to social integration."
David M. Semple, MBBS, Consultant Psychiatrist at Hairmyres Hospital, Andrew M. McIntosh, MSc, MD, MPhil, MBChB, Clinical Reader in Psychiatry at the University of Edinburgh, and Stephen M. Lawrie, MD, MPhil, MBChB, Professor of Psychiatry and Neuroimaging at the University of Edinburgh, stated the following in their Mar. 2005 article "Cannabis as a Risk Factor for Psychosis: Systematic Review," published in the Journal of Psychopharmacology:
"Meta-analysis suggests that cannabis is a risk factor, increasing the chances of developing schizophrenia or a schizophrenia-like psychotic illness by approximately three-fold...
Our findings underline the need to recognize the use of cannabis as a significant risk factor for schizophrenia and schizophrenia-like psychotic illness."
Louisa Degenhardt, PhD, Senior Lecturer at the National Drug and Alcohol Research Centre at the University of New South Wales, and Wayne Hall, PhD, Professor at the School of Population Health at the University of Queensland, stated in their article "Is Cannabis Use a Contributory Cause of Psychosis?" published in Aug. 2000 in the Canadian Journal of Psychiatry:
"Results: Evidence from 6 longitudinal studies in 5 countries shows that regular cannabis use predicts an increased risk of a schizophrenia diagnosis or of reporting symptoms of psychosis.
The relation did not seem to be a result of cannabis use to self-medicate symptoms of psychosis.
Conclusions: It is most plausible that cannabis use precipitates schizophrenia in individuals who are vulnerable because of a personal or family history of schizophrenia."
David M. Fergusson, PhD, Research Professor in the Department of Psychological Medicine at the Christchurch School of Medicine & Health Sciences at the University of Otago in New Zealand, et al., wrote in their article titled "Tests of Causal Linkages Between Cannabis Use and Psychotic Symptoms," published in Mar. 2005 in Addiction:
"Daily users of cannabis had rates of psychotic symptoms that were between 1.6 and 1.8 times higher than non-users of cannabis....
The results of the present study add to a growing body of evidence suggesting that regular cannabis use may increase risks of psychosis.
The present study suggests that:
the association between cannabis use and psychotic symptoms is unlikely to be due to confounding factors; and
the direction of causality is from cannabis use to psychotic symptoms."
Andrew Johns, MB, Consultant in Forensic Psychiatry at the Maudsley Hospital in London, noted in a Feb. 2001 article, "Psychiatric Effects of Cannabis," published in the British Journal of Psychiatry:
"There is good evidence that taking cannabis leads to acute adverse mental effects in a high proportion of regular users. Many of these effects are dose-related, but adverse symptoms may be aggravated by constitutional factors including youthfulness, personality attributes and vulnerability to serious mental illness....
An appreciable proportion of cannabis users report short-lived adverse effects, including psychotic states following heavy consumption, and regular users are at risk of dependence...
The untoward mental effects of cannabis may be classified:
Psychological responses such as panic, anxiety, depression or psychosis. These effects may be described as 'toxic' in that they generally relate to excess consumption of the drug.
Effects of cannabis on pre-existing mental illness and cannabis as a risk-factor for mental illness.
Oakley Ray, PhD, Emeritus Professor of Psychology and Pharmacology at Vanderbilt University, and Charles Ksir, PhD, Professor Emeritus of Psychopharmacology and Drug Policy at the University of Wyoming, wrote in their 2004 textbook Drugs, Society and Human Behavior:
"There have been reports of psychotic 'breakdowns' occuring with rare frequency after marijuana has been smoked, but the causal relationship is in question.
The psychotic episodes are generally self-limiting and seem to occur in individuals with a history of psychiatric problems."
Martin Frisher, PhD, Senior Lecturer in Health Services Research at Keele University, et al., stated the following in their Sep. 2009 article titled "Assessing the Impact of Cannabis Use on Trends in Diagnosed Schizophrenia in the United Kingdom from 1996 to 2005," published in Schizophrenia Research:
"The results of this study indicate that the incidence and prevalence of diagnoses of schizophrenia and psychoses in general practice did not increase between 1996 and 2005...
This study does not therefore support the specific causal link between cannabis use and the incidence of psychotic disorders...
The most parsimonious explanation of the results reported here are that the schizophrenia/psychoses data presented here are valid and the causal models linking cannabis with schizophrenia/psychoses are not supported by this study."
Colin Blakemore, PhD, ScD, Chair of the Department of Physiology at the University of Oxford, wrote in a Dec. 27, 2002 email to ProCon.org:
"It is conceivable that excessive use of cannabis sometimes contributes to acute schizophrenic episodes. But it is difficult to believe that cannabis is a strong risk factor for this disorder, because there is no evidence that the incidence of schizophrenia has risen dramatically over the past 50 years, in parallel with the huge increase in cannabis use.
Young schizophrenic patients are often heavy cigarette smokers too, but no-one would suggest that tobacco causes schizophrenia."
Jason Schiffman, PhD, Associate Professor of Clinical Psychology at the University of Hawaii at Manoa, et al, reported in their article, "Symptoms of Schizotypy Precede Cannabis Use," published Mar. 30, 2005 in Psychiatric Research:
"The onset of schizotypal symptoms generally precedes the onset of cannabis use. The findings do not support a causal link between cannabis use and schizotypal traits."
The Institute of Medicine published in its Mar. 1999 report titled "Marijuana and Medicine: Assessing the Science Base":
"Although euphoria is the more common reaction to smoking marijuana, adverse mood reactions can occur. Such reactions occur most frequently in inexperienced users after large doses of smoked or oral marijuana. They usually disappear within hours and respond well to reassurance and a supportive environment."
Paul Armentano, Senior Policy Analyst at the National Organization for the Reform of Marijuana Laws (NORML), stated in an Aug. 2, 2007 press release titled "NORML Responds to New Rash of Pot and Mental Health Claims":
"Despite the enormous popularity of cannabis in the 1960s and 1970s in numerous Western cultures, rates of psychotic disorders haven't increased since then in any of these societies. Individuals suffering from mental illness such as schizophrenia tend to use all intoxicants - particularly alcohol and tobacco - at greater rates than the general population. Not surprisingly, many of these individuals also use cannabis."
Hollie V. Thomas, DPhil, Director of the MSc Program in Psychiatry at Cardiff University, wrote in an article titled "Psychiatric Symptoms in Cannabis Users," published Aug. 1993 in the British Journal of Psychiatry:
"Cannabis use can lead to a range of short-lived symptoms such as de-personalisation, de-realisation, a feeling of loss of control, fear of dying, irrational panic and paranoid ideas...
The evidence that cannabis has a causative role in chronic psychotic or affective disorders is not convincing, although the drug may modify the course of an already established illness."
Lynn Zimmer, PhD, late Professor Emeritus at the Queens College, noted in her 1997 book Marijuana Myths, Marijuana Facts:
"Given that the incidence of schizophrenia declined substantially in Western societies in the 1970s, at the same time cannabis use was rising, it seems highly unlikely that marijuana causes schizophrenia in otherwise healthy people....
Cannabis psychosis is self-limiting, disappearing in a few days with or without medical treatment. Toxic psychosis probably occurs more commonly in individuals with preexisting psychiatric disorders....
Marijuana temporarily alters mood, thought, emotions, and perception, sometimes quite dramatically. None of marijuana's effects cause people to behave in any particular manner. In the midst of a toxic psychosis, people may become agitated and frightened. In response to acute panic, people may become withdrawn and inactive. Neither of these states eliminates the social and moral restraints that guide human behavior."