“Smoking marijuana as a teenager lowers IQ for life, scientists warn,” the Mail Online reports. The headline is prompted by a critical review looking at the evidence about the potential harms associated with cannabis use.
This latest review was written by researchers from the US National Institute on Drug Abuse and provides an overview of the potential harms associated with cannabis use, ranging from increased risk of car accidents to an adverse effect on “life achievement”.
The Mail’s coverage of the story was an accurate representation of the research, although it took the review findings at face value and did not mention any of its limitations. The most important of which is that it does not appear to be a systematic review, where all available evidence on a particular topic is assessed.
It is unclear whether this study was prone to “cherry picking” – where evidence that supports the researchers’ arguments is included while evidence that opposes it is ignored. This has the potential to bias the findings and conclusions.
The timing of the study is also interesting. After the quasi-legalisation of cannabis in the states of Colorado and Washington there have been increasing calls to roll-out similar laws across America.
Many of the review's conclusions were tentative or indicated more research was needed. Based on this review alone, research into the effects of cannabis use in humans has not yet yielded any firm conclusions and often paints an unclear or inconsistent picture.
The authors state cannabis (marijuana) is the most commonly used illicit drug in the US with around 12% of people over the age of 12 reporting use in the past year, with particularly high rates among young people.
The regular use of cannabis in adolescence was of particular concern to the review group because of a higher chance of potential harm in younger groups.
The review highlighted a popular belief of cannabis as a harmless vice. Given the loosening of laws and regulation on recreational use in in some US states (Colorado and Washington), some may see this as giving legitimacy to this idea. And in response to the increase in the legalisation of cannabis for medical and recreational use in the US, patients may be more likely to ask their doctors about its potential benefits and risks to health.
The authors indicated many scientific studies report harmful effects associated with cannabis but others do not, which has led to heated debate about whether cannabis is harmful.
The review aimed to assess the current state of the science on the adverse health effects of recreational cannabis use, focusing on those areas for which the evidence is strongest.
The review was broad, covering addiction, neurodevelopment, mental illness, risk of cancer and life chances.
The authors indicated that, “despite some contentious discussions regarding the addictiveness of marijuana, the evidence clearly indicates that long-term marijuana use can lead to addiction”. Around 9% of those who experiment with cannabis become addicted, and this figure rises to 50% of those who smoke it every day, the review said.
The negative effects of cannabis use on the brain were said to be particularly prominent if use starts in adolescence or young adulthood. The review flagged up one study that found an association between frequent use of cannabis from adolescence into adulthood with significant declines in IQ.
There is a theory that using cannabis in early adulthood could encourage the use of other addictive drugs such as nicotine or alcohol. The review found animal studies that were consistent with this theory. However, they also acknowledged they didn’t know whether people with more addictive tendencies are more likely to try cannabis, alcohol and nicotine, or whether cannabis use itself directly increased the chances of them trying these things.
It has also been linked to psychosis (including those associated with schizophrenia), especially among people with a pre-existing genetic vulnerability, and worsens illness in people with schizophrenia.
The authors say it is difficult to establish causality in these types of studies, because factors other than cannabis use may be directly associated with the risk of mental illness. Other factors could also predispose a person to both cannabis use and mental illness. This makes it difficult to confidently attribute the increased risk of mental illness to cannabis use.
The review indicated that cannabis use impairs critical cognitive functions, both during acute intoxication and for days after use. For this reason, students who smoke cannabis could be functioning at a cognitive level that is below their natural capability for considerable periods of time. However, the results from studies included in the review were inconsistent.
Some studies suggested the long term negative effects of cannabis use on learning may be reversible, whereas others indicated memory and attention got significantly worse the more years someone had been using cannabis regularly.
Some studies suggested early cannabis use is associated with impaired school performance and an increased risk of dropping out of school, but there were other factors that could explain this link. For example, shared environmental factors like growing up in a neighbourhood where drug use is high and academic achievement low.
There were studies indicating that both immediate exposure and long-term exposure to cannabis impair driving ability. According to a meta-analysis, the overall risk of involvement in an accident increases by a factor of about two when a person drives soon after using cannabis. The risk associated with the use of alcohol in combination with cannabis was reported to be greater than that associated with the use of either drug alone.
The effects of long-term cannabis smoking on the risk of lung cancer are unclear and often complicated by the fact many cannabis smokers also smoke tobacco – which is known to cause cancer. Separating the health effects of the two types of smoking remains a challenge. The researchers warned that the strength of cannabis in circulation is increasing and so any potential risks may also be increasing.
The review’s main weakness is that it didn’t describe how it searched and reviewed the evidence on this subject and whether this was systematic. This means that key evidence may have been missed, leading the authors to potentially draw biased conclusions. It is not possible to say that the results were biased; only that without the methods, there remains a risk that they are.
A second limitation is that the evidence behind the review’s conclusions was drawn largely from early animal studies, or from observational studies that couldn’t establish cause and effect. This means we can’t really make clear cut, definitive statements about the health effects of cannabis use as the evidence, at least the evidence identified in this review, was not strong in most areas. The long term effects of cannabis use, for example, remain in the authors’ words “poorly understood”.
The review generally highlighted a scarcity or lack of knowledge on the beneficial or harmful effects of cannabis. This may be a true reflection of the evidence base, or may be in part because the review has not included all the relevant evidence.
One of the biggest challenges in studying the effects of cannabis is that due to its legal status in much of the world, researchers could not legally use cannabis in the “gold standard” of studies – a randomised controlled trial. There could also be ethical problems in randomly assigning people to use cannabis who don’t already, given its potentially damaging effects.
As more and more parts of the world are now legalising (or at least partially decriminalising) cannabis, research of this type could potentially be carried out and may allow us to learn more about the benefits and risks of this widely used drug.