Will Legalization and Commercialization of Cannabis Use Increase the Incidence and Prevalence of Psychosis?

Cannabis legalization and commercialization are increasing worldwide, with rising tetrahydrocannabinol potency linked to greater risks of dependence and psychosis. Dependence risk has risen from 9% to near 30% as potency increased. Daily use of high-potency cannabis raises psychosis risk up to 9-fold, comparable to the lung cancer risk from smoking 30 cigarettes daily. Evidence from prospective studies shows a causal relationship between cannabis use and later schizophrenia-like psychosis, not explained by confounding factors or reverse causation. Adolescents and those with a family history of psychosis are especially vulnerable. The cannabis industry's marketing and lobbying echo tactics used by the tobacco industry to downplay health harms.

Study at a glance

Abstract

There is a worldwide trend toward liberalizing cannabis policy and commercializing its sale. Uruguay legalized recreational cannabis in 2013, as did Canada in 2018, as well as 10 US states.1 Other countries have decriminalized the drug. In Holland, cannabis can be bought in designated cafes, and in Portugal, the police refer those who regularly use cannabis for counseling. Psychiatrists have played a prominent role in the debate over the health consequences of legalization in many countries, especially in the UK, but the public debate in the US has been notable for the absence of input from psychiatrists.2 Tetrahydrocannabinol is responsible for the “high” that those who use cannabis enjoy. The proportion of tetrahydrocannabinol in cannabis has risen from 3% or less in many traditional herbal forms to an average of 10% to 15% in Europe and North America.1,3 In Colorado, the first state to legalize recreational use, potency has spiraled upwards of 70% with tetrahydrocannabinol content in shatter (solid cannabis extract) and wax dabs (soft, honeylike cannabis extract). In those US states that have legalized cannabis, the price has fallen1 and both cannabis use and dependence have increased among adults.1,3,4 The risk of dependence among those who use cannabis was estimated at 9% in the early 1990s but has increased along with potency, with 1 report suggesting it is now closer to 30%.3 Unintentional overdoses and driving while intoxicated have significantly increased. There is concern about the effects of cannabis on the unborn child, particularly since many women appear to use cannabis during pregnancy to counteract nausea.5 Those who actively use cannabis show cognitive impairment while they are intoxicated, but it is unclear whether the effects persist after they stop using cannabis. There are some reports that cannabis use increases risk of depression and suicide,6 but by far the strongest evidence concerns psychosis. Numerous prospective studies have shown that cannabis use carries an increased risk of later schizophrenia-like psychosis.2,6 Sideli et al6 state that “...systematic reviews and methodologically robust studies have confirmed the important role of cannabis use in the initiation and persistence of psychotic disorders.” Indeed, of 13 prospective longitudinal studies, 10 showed that those who use cannabis had a significantly increased risk of psychosis compared with those who do not, while 2 of the remaining 3 showed a trend in the same direction.6 Cannabis potency is considered high if it contains greater than 10% tetrahydrocannabinol. There is a doseresponse relationship in those who use high-potency cannabis moderately (the risk doubles), heavily (it increases 5-fold), and daily (it increases up to 9-fold).7 The latter relative risk is similar to the risk of lung cancer in those who smoke 30 cigarettes per day. Those individuals who have a family history of psychosis and those who begin using cannabis in adolescence (which is only too common) appear especially vulnerable. Patients diagnosed with psychosis who continue to use cannabis have a worse prognosis than those who cease to use it, with more frequent episodes and longer periods of hospitalization for their illnesses.6 Critics have proposed a number of alternative explanations to the causal hypothesis, but one by one, these have been rebutted.6 Confounding by other drug use or by prior psychological deviancy does not negate the effect of cannabis.6 It was hypothesized that patients may use cannabis to counteract symptoms of psychosis, or even allay anxiety in the prodrome, but empirical data do not support this hypothesis.6 A more pertinent criticism of the causal hypothesis is that those who are genetically predisposed to schizophrenia might be more likely to use cannabis than the rest of the population. Some studies suggest that a small proportion of variance in initiating cannabis use may be explained by such genetic predisposition—others disagree. However, if there is any effect, it is small and cannot explain the strong association between the daily use of high-potency cannabis and later psychosis.6 It is often forgotten that tobacco was initially considered a harmless, natural plant. Health problems only came to the fore after the tobacco industry invented cigarettes for ease of use, mass-produced them, and marketed them using persuasive advertising. Cigarette prices plummeted and governments were seduced by the employment and tax income generated. A skillful campaign even persuaded many psychiatrists that smoking cigarettes could counteract the cognitive impairment in schizophrenia. In a similar manner, the legalization of cannabis production and sale has created a rapidly growing industry with a strong financial interest in promoting cannabis use. As a result, commercialization of cannabis is now rampant, and a dazzling array of cannabis products in the form of ice cream, cakes, and sweets is available. A recent report projected annual legal sales of cannabis at $66.3 billion by 2025. Tobacco and alcohol companies are buying into the industry.3 Sophisticated lobbying and marketing by the cannabis industry highlights the tax revenue for governments and the health benefits of “medical use.” As with the first reports more than 60 years ago of a causal association between cigarette smoking and lung cancer, the cannabis industry denies or minimizes the evidence that cannabis use increases the risk of psychosis. The long-term adverse effects of tobacco and alcohol track the extent of their use, both of which are VIEWPOINT