Here's Why We Hear So Many False Claims About Cannabis

09/15/2015 12:16 EDT | Updated 09/15/2016 05:12 EDT
Shutterstock / Elena Ioachim

By: David Nutt, DM, Edmond J Safra Professor of Neuropsychopharmacology, Division of Brain Sciences, Imperial College London and Founder of DrugScience

As a neuropsychopharmacologist at Imperial College London, I study the impact of drug use on the brain. Like many scientists studying these issues, the results I detect in my lab have direct implications on how we can better equip ourselves as a society to deal with drugs.

On the whole, my many years of research on substance use has taught me a major overarching lesson: we are much more likely to demonize drugs for their negative effects than consider their neutral or potentially positive impacts. Or -- in scientific terms -- there is a built-in bias in the scientific literature, textbooks, and popular press towards highlighting the negative aspects of drug use. And more ink has been spilled about cannabis than any other drug, perhaps because it's the most widely used illegal drug and the subject of intense debate concerning its regulation.

Because my research focuses on neurological function, I often come across claims about cannabis' impact on the brain. You've likely heard them too: claims that cannabis use lowers IQ by up to eight points, that use of the drug causes schizophrenia, and that it impairs cognitive function in the long term. What's fascinating about these claims is that they're almost always "based on the scientific evidence." But is that really the case?

A recent set of reports by the International Centre for Science in Drug Policy (note: I act as a scientific board member for the organization) addresses this question head-on by evaluating the science in support of some of the most high-profile claims about cannabis. The results are compelling: taken together, they demonstrate how easily evidence can be twisted to support political ends.

A recent example, often-repeated, is that cannabis causes drops in IQ, of up to eight points -- which was the major finding of a 2012 report from Duke University. But, these results were based only on one study and the 8-point drop in IQ was observed in only 38 individuals (3.7 per cent of the total sample). Just last year, a University College London report was published that re-examined this issue in another group of young people, with a sample size over twice as large as the Duke study, and taking into account alcohol use, cigarette use, maternal education, and other potential confounders. The result? The authors found no association between cannabis and IQ at all, not even among heavy cannabis users. It is clear that we are some way off from a final answer to the question of whether cannabis is ever harmful to intelligence.

So given the conflicting findings on this issue, why does this claim continue to circulate as if it were unequivocally true? Part of it has to do with our appetite for discovery: our society is wired, for better or for worse, to celebrate and obsess over technological and scientific breakthroughs. So when a new scientific finding is made, particularly about a drug like cannabis that so many people are familiar with, it makes headlines. Unfortunately, our collective interest in the issue wilts away and the corrections and conflicting findings that come a year, two years or five years later don't get nearly as much attention.

The other reason that false claims about cannabis continue to circulate is a little more insidious. It boils down to the trust we place in our gut feelings and the way that we've been educated about drug use in general. Using drugs like cannabis, we've been taught, makes you stupid. So when we see a headline touting scientific evidence inconsistent with how we've been educated to feel about cannabis, instead of changing our minds based on the evidence, it is much easier to ignore it. The end result is too often that the many scientific corrections, refutations, and re-interpretations of the evidence just get swept under the carpet. We don't want to believe we have been mislead for all these years.

There's one more important reason, I believe, that reporting on cannabis science tends towards the negative: money. Scientists examining the health and social impacts of drugs are usually funded by government agencies, and they often highlight the negative effects of drugs to justify their own source of funding. If a scientist can show that a drug is harmful, then they can show that it's important to do more research on the topic to protect society. By contrast, if use of a drug appears to have only benign effects, then why would the government bother spending more research money on it? This cycle has perpetuated a scientific industry intent on demonstrating the harmful effects of drugs like cannabis, precisely because doing so allows the industry to justify its own existence. Of course, one of the major consequences of this cycle is that research on the potential benefits of drugs gets sidelined, for example with not nearly enough government funding for studies on cannabis' health benefits.

There is a massive and growing scientific literature on cannabis. And yet, for all its complexities and contradictions, we only tend to hear about the most negative study outcomes. When people state with conviction that cannabis causes a loss of IQ points, or causes schizophrenia, they are wrong in their certainty. The evidence on these questions is ambiguous and contradictory. Whilst the evidence accumulates, we need to remain open-minded to the possibility that cannabis does not impact IQ or cause schizophrenia, just as we should remain open-minded to the likelihood that heavy cannabis use in adolescence can cause serious harm to some. Understanding the scientific evidence on these issues, which the ICSDP's new reports help us to do, is the first step towards having an informed discussion about how we should treat cannabis. The harder part is understanding and overcoming the many obstacles that restrict the use of that same evidence in our public discourse.


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