Existing research suggests that people who use cannabis may be more likely than non-users to use and opioids and develop opioid use disorders (OUD). The corresponding study was published in Addiction by researchers in the UK and Australia.
Cannabis is well-known as a gateway drug for other, more harmful drugs, with its reputation having inspired drug policies around the world. However, few studies have actually investigated whether this is true.
Moreover, those that have investigated the link are typically of low quality as they do not account for enough variables that could affect cannabis and drug use.
To make a consensus of existing research on the subject, the researchers conducted the first-ever systematic review and meta-analysis on the likelihood of cannabis being a gateway drug for first-time opioid use, OUDs, and dependence or abuse.
All in all, they included just six studies as others on the topic were not of high enough quality to extract meaningful results. The studies included contained data from 102,461 participants from the US, Australia, and New Zealand. All of the data was collected between 1977 and 2017.
In the end, the researchers found that cannabis users may be more than twice as likely than non-users to begin using opioids and develop problematic usage patterns.
The researchers note, however, that all of the studies analyzed had a moderate risk of bias. This is because they missed important confounding variables such as frequency of cannabis use and affiliation with peers who use cannabis or opioids.
The researchers thus conclude that, while it is unclear whether the unmeasured variables would have significantly influenced the results, it is currently not possible to say whether there is a causal relationship between cannabis use and opioid use. They say, however, that the two are likely to be at least partially linked.