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The idea that cannabis is a ‘gateway drug’ to more harmful substances such as opioids is controversial, yet has substantially impacted drug policy, education and how we conceptualize substance use. A new systematic review and meta-analysis has found that people who use cannabis are disproportionately more likely to initiate opioid use and engage in problematic patterns of use than people who do not use cannabis. But the quality of the evidence for this finding is low.
One surprising discovery in this meta-analysis, led by researchers at the University of Sydney’s Matilda Centre for Research in Mental Health and Substance Use and published by the journal Addiction, was that there aren’t many good studies on the gateway drug theory. Despite the frequency with which the topic of gateway drugs is discussed, only six studies were of high enough quality to include. A synthesis of the evidence from those six studies demonstrated that people who use cannabis are more than twice as likely to initiate opioid use and develop problematic patterns of use than people who don’t use cannabis.
But the quality of the evidence in those studies is low and must be interpreted with caution. All six studies had a moderate risk of bias and overlooked important confounding variables such as cannabis use frequency and affiliation with cannabis or opioid-using peers. It is unclear whether these unmeasured variables would have had a strong enough impact to explain away the cannabis-opioid use relationship.
It is therefore not possible, on the existing evidence, to state conclusively that there is a causal relationship between cannabis and subsequent opioid use, but it is likely that there is at least a partial causal relationship.
The six studies provided data from the United States, Australia and New Zealand between 1977 and 2017, with a total sample of 102,461 participants.
Society for the Study of Addiction