Does Cannabis Cause Schizophrenia? | psychology today

Many states now make marijuana available for recreational use. Unfortunately, popular literature and the Internet are littered with misinformation and prejudice regarding whether marijuana causes psychosis. Does Cannabis Cause Schizophrenia? Some recent studies indicate that the answer depends on three factors. It depends on your genes, your age and how much THC, the main active ingredient, is absorbed by the body. Consider each condition.

Who are your parents?

Inheriting a genetic predisposition to psychosis makes a person vulnerable to the consequences of long-term, high-dose cannabis use. Unfortunately, there is currently no genetic test that can tell a person with certainty that they are at risk of developing psychosis. Your only option is to consider your siblings: do any of them seem psychotic to you? If so, then you should seriously reconsider using marijuana.

Despite reports from observational studies of a dose-dependent correlation between cannabis use and risk of psychosis, a causal link between cannabis use and psychotic disorders remains unproven. Why? Correlation results are often hampered by the possibility that another risk factor associated with cannabis use is more closely associated with the development of psychosis. Alternatively, those affected by the early symptoms of schizophrenia could turn to marijuana for self-treatment. Cannabis use by these nascent psychiatric patients could reduce some of their symptoms long before they become part of their diagnosis.

A study recently published in the journal Molecular Psychiatry (Vol 23, 1287-1292) identified 10 different genetic variants, called single nucleotide polymorphisms (SNPs), in nearly 80,000 subjects. The risk of schizophrenia was increased if these variants were present in marijuana users. A word of caution when considering your risk: Simply having a SNP that puts you at risk is not enough to induce psychosis, should you decide to use marijuana. The reality is much more complicated than one gene = one disease.

The link between marijuana and schizophrenia is also blurred by the fact that cannabis use is strongly associated with tobacco use. Conversely, tobacco use is strongly correlated with marijuana. Indeed, tobacco can act in synergy with marijuana to create addiction. Worse still, tobacco use increases the risk of becoming schizophrenic (not much is read about this link in the popular press). Overall, current genetic evidence indicates that the association between marijuana and psychosis is significantly influenced by tobacco use. This association essentially undermines any direct genetic link between marijuana use and schizophrenia, for example, if a person smokes, then it becomes impossible to establish a causal link between cannabis and psychosis. So, marijuana users who also smoke cigarettes face a difficult dilemma: do they feel genetically lucky?

How old are you?

The prenatal brain is very vulnerable to the presence of cannabis. Pregnant women should never use cannabis.

Are you a teenager? The available evidence indicates that the adolescent brain is still vulnerable to exogenous cannabinoids. Essentially, cannabis alters the normal trajectory of brain maturation, although the consequences are less severe than those seen after prenatal exposure. The combined evidence from numerous human and animal studies suggests that cannabis exposure during adolescence has the potential to produce subtle, but long-lasting alterations in brain function and behavior. Severity differs depending on duration of use, age at first use, and underlying genetic vulnerabilities that are more likely to appear in adolescence, such as various psychopathologies.

If you are past middle age, the use of whole herb medical cannabis does not negatively impact cognition in older patients. Why do the effects of cannabis depend on age? The answer to this question remains unknown, but there is a fascinating potential mechanism that involves what happens to your brain stem cells. Stem cells undergo neurogenesis and give rise to new neurons every day. Neurogenesis is obviously essential for the developing brain from the prenatal period through adolescence. Neurogenesis begins to decline around middle age and is almost absent by the time you are ready to retire.

Numerous PET imaging studies in humans suggest that the decline in neurogenesis is due to increased levels of brain inflammation that occur naturally with age. The decline in neurogenesis likely underlies age-related learning and memory impairment and an increased incidence of depression. Research in my lab has shown that a daily low dose of cannabis can significantly reduce inflammation in the brain. Research from my lab has also demonstrated that stimulation of cannabis receptors on surviving stem cells can restore neurogenesis.

How much do you smoke each day and what do you smoke?

Cannabis products available today contain much higher concentrations of THC than ever before. Most of the available epidemiological data becomes irrelevant when we try to compare data collected over the past decades. Dosage matters. How often someone uses marijuana each day also matters. Some of the original studies on the link between marijuana and psychosis found that heavy users were more likely to develop symptoms of psychosis. Some of these “heavy users” reported smoking more than 100 marijuana cigarettes per day. This is obviously not typical for most users. The problem is that the actual dose consumed is often difficult to determine in epidemiological studies.

Essential readings on psychosis

Today, medical science lacks sufficient data to definitively say whether smoking cannabis induces psychosis. If you are young, use a lot of high-dose cannabis products every day, and have a genetic predisposition to psychosis, then yes, you are at risk. Numerous studies over the past decade have demonstrated that people who use cannabis and progress to schizophrenia have a significantly higher familial risk of psychosis compared to people who use cannabis but do not progress to schizophrenia.

So if you are middle-aged, occasionally use marijuana, and have no known genetic predisposition, then no, you are not at risk. This last condition, your genetics, is the wildcard in your genetic package since science still does not have the ability to define the genetics of psychosis. Despite the lack of certainty, it is very likely that many people will go ahead and spin the wheel of genetic roulette.

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