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4 Cannabis Claims Debunked

The forces fighting to preserve prohibition make countless claims about cannabis, but how many of these attacks are supported by the facts? We explore four of the most common myths—cannabis reduces IQ points, increases schizophrenia risk, makes you lazy and will spread like wildfire once legal—and see if there is any truth to the propaganda.

Myth #1: Cannabis Lowers IQ

Proceedings of the National Academy of Sciences of the United States of America—otherwise known as the clinical journal with the long-ass title—published a study in 2012 that suggested “persistent cannabis users show neuropsychological decline from childhood to midlife.” The Duke University researchers utilized data from the Dunedin Study of 1,037 individuals from birth (1972 or 1973) to age 38, and they found that “persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education.” Likewise, the study concluded, “Cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users. Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain.”

In other words, smoking blunts causes a permanent drop in your IQ.

The study quickly picked up traction, and one of the many references included a Washington Post columnist on Meet the Press. In her argument against full cannabis legalization, she said, “The best evidence is that you lose—if you use marijuana as a teenager regularly—eight IQ points."

The columnist apparently missed the update. A few months after the study, the same journal published a refutal of the study’s findings.

“Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status”—published in January 2013 in the same journal—said this about the previous findings: “Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature.”

In a nutshell, the 2013 refutal said the earlier study did not account for confounders, that is, other circumstances that might account for lower IQs. In particular, the new study suggested “an alternative confounding model based on time-varying effects of socioeconomic status on IQ.” The study added that “heavy, persistent, adolescent-onset cannabis use involves a culture and norms that raise the risk of dropping out of school, getting entangled with crime, and other such behaviors. Unlike a neurotoxic effect, however, this effect would be nonpermanent and mediated by the cognitive demands of different environments.”

In a press release for the 2013 refutal, the lead author said, “This is a potentially important public health message: The belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviors.”

A reviewer at Oxford University seconded this notion, stating, “The current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself.”

In 2017, the Addiction journal published a study that looked at 1,989 twins involved in the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative birth cohort of twins born in England and Wales from 1994 to 1995. Once again, the findings showed no evidence of neuropsychological decline. 

"Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence," the authors wrote. "Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests."

What made the Addiction study special? It was the same lead researcher who made the original claim in 2012. She made the bold move of reversing herself and admitting her original 2012 claim was wrong. Now let's hope the rest of the anti-cannabis community will do the same. 

Myth #2: Cannabis Causes Schizophrenia

In 1987, a research team published “Cannabis and Schizophrenia: A Longitudinal Study of Swedish Conscripts” in The Lancet journal that suggested “cannabis is an independent risk factor for schizophrenia.” More research followed, including a 2004 British Journal of Psychiatry study that said ending all cannabis use would decrease incidence by eight percent worldwide. The findings also suggest that adolescent use in particular increases risk. Support for this myth, however, is not holding up.

First, consider the anecdotal evidence. In 1969, the Gallup poll first started asking questions about illegal drugs, and only four percent of American adults in that survey had ever tried cannabis. Alana Anderson, a college graduate quoted in that 1969 poll, said, “My generation was told that marijuana caused acne, blindness and sterility.” The percentage of people who tried cannabis grew to 24 percent by 1977 and 33 percent by 1985. In the 2015 Gallup poll, 44 percent admitted to trying cannabis, and 58 percent wanted to see it legalized.

The number of people who tried cannabis increased elevenfold between 1969 and 2015, during which time cannabis potency also increased significantly. If cannabis increases the risk for developing schizophrenia, surely the rate of incidence increased proportionally to the rise in cannabis consumption. In reality, schizophrenia rates remained steady (1.1 percent of American adults), and the 1995 Schizophrenia Bulletin study “Time Trends in Schizophrenia” said rates actually seem to be in decline over the past few decades.

Now for the clinical evidence. Several recent studies have not found a cannabis-schizophrenia connection, and more importantly, they found no connection between adolescent use and schizophrenia. For example, a 2005 study in Biological Psychiatry found that “cannabis use had no such adverse influence” and that “the vast majority of young people who use cannabis do not develop psychosis.” The 2015 study “Chronic Adolescent Marijuana Use as a Risk Factor for Physical and Mental Health Problems in Young Adult Men” in Psychology of Addictive Behaviors also found no connection. Lead researcher Jordan Bechtold explained in a press release, “There were no differences in any of the mental or physical health outcomes that we measured regardless of the amount or frequency of marijuana used during adolescence.”

In 2011, Archives of General Psychiatry published “Cannabis Use and Earlier Onset of Psychosis” that argued in favor of the cannabis-schizophrenia connection noting that symptoms typically manifest 2.7 years earlier for those who consume cannabis. Still, the authors admitted, “Not all researchers agree that the association between cannabis use and earlier age at onset is causal.” Some studies “argue that the association between cannabis use and earlier age at onset could be explained by demographic variables, including lower socioeconomic status and the proportion of male cannabis users” and that “the apparent association between earlier age at onset and cannabis use might simply be owing to older patients with first-episode psychosis being less likely to use cannabis.”

If the basis for this argument is that symptoms manifest 2.7 years earlier in people who consume cannabis, there might be an even simpler explanation. As any addiction therapist can attest, mental health disorders are leading drivers for substance abuse, and many people turn to illicit and prescription drugs to self-medicate symptoms. During the early stages of schizophrenia, many people might turn to alcohol, cannabis or prescription drugs to mute the symptoms. In other words, schizophrenia is the risk factor for cannabis use, not the other way around.

Myth #3: Cannabis Makes You Lazy

When the media depicts a cannabis consumer, it is more likely to show the person on the couch eating potato chips than creating art, exercising or working hard. The stigmatizing myth behind this characterization is that “stoners” are lazy. No one would describe people like Richard Branson, Steve Jobs, President Obama and Oprah Winfrey as lazy, but the prohibitionists would likely argue that these current/past stoners are outliers. A team of three clinical researchers (including Dr. Mitch Earleywine) decided to find out.

In 2006, the researchers published “Cannabis, motivation and life satisfaction in an internet sample” in the clinical journal Substance Abuse Treatment, Prevention and Policy. Setting up the premise, the authors wrote, “Although little evidence supports cannabis-induced amotivational syndrome, sources continue to assert that the drug saps motivation, which may guide current prohibitions. Few studies report low motivation in chronic users; another reveals that they have higher subjective wellbeing.”

The study surveyed 1,300 individuals, including 243 who consume cannabis daily and 244 who fully abstain. Nearly 31 percent of the cannabis consumers cited medical intent in treating conditions like chronic pain and nausea, issues that could negatively affect motivation levels. Directly comparing the chronic and non-users, the study still found that motivation levels were the same.

The researchers concluded, “In this study, participants who used cannabis seven days a week demonstrated no difference from non-cannabis users on indices of motivation. These findings refute hypothesized associations between heavy cannabis use and low motivation. Means and median comparisons, data transformations, and robust comparisons of central locations each revealed no statistically significant differences among motivation scores despite improvements in power.”

These findings beg the question: Why were the prohibitionists so lazy as to create a myth and stereotype without doing the research to see whether or not it was true?

Myth #4: Legalization Increases Cannabis Use

A common myth is that legalizing cannabis will increase overall use. If we are talking about safe and responsible adult use, this should not matter. Still, the data does not show a direct link between legalization and usage rates. For example, the Netherlands celebrates 40 years of de facto legalization next year, yet their cannabis usage rates are lower than that of the United States.

In 2015, the International Centre for Science in Drug Policy (ICSDP) published the 41-page State of Evidence: Cannabis Use and Regulation that said legalization “has at most a marginal impact on the prevalence of drug use.” As evidence, the researchers looked at the World Health Organization’s 17-country, 85,052-participant World Mental Health Surveys. The U.S. and New Zealand, both countries that criminalize cannabis, had the highest usage rates. The authors also found that any form of legalization, including medicinal, did not trigger an increase in usage.

However, studies like the 2012 “Medical Marijuana Laws and Teen Marijuana Use” found that teenage cannabis use decreased in states that instituted medical cannabis laws. One of the authors, Dr. Benjamin Hansen, said potential causes for decrease might include a limited illegal supply “as some drug dealers shift over to legally supply through dispensaries. Although total supply might be going up, the supply available to youth might decrease [as suppliers] start up legal dispensaries and don’t want to risk their business by selling to someone underage. Second, more adults might be using marijuana due to the decreased punishments and the availability of medical marijuana. In previous research, we found that young adult use of medical marijuana increased by nearly 25 percent in several states for which we had data. An increase in adult demand would drive prices up, which would result in a decrease in quantity which teens demand.”

Co-author Dr. Daniel Rees added, “The argument that medical marijuana is responsible for the recent increases in the use of marijuana by teenagers is not backed up by the data. If teen use of marijuana had gone up in the years following legalization, we would have observed a correlation. No effect, no correlation.”

Countries differ, and evidence suggests that legalization does not always (or even commonly) correspond with an increase in usage. However, in the U.S., evidence suggests that teen use might decrease.

Myth #3: Cannabis Makes You Lazy

Myth #2: Cannabis Causes Schizophrenia

Myth #1: Cannabis Lowers IQ

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