The relationship between cannabis and tobacco use - a systematic review and meta-analysis

Background: Cannabis and tobacco use commonly co-occur and understanding their relationship can help inform public health strategies to prevent their harms. We conducted a systematic review and meta-analysis to examine the prospective relationship between cannabis and tobacco use.

Methods: We conducted a systematic review and meta-analysis. PubMed, Embase, PsycINFO, Google Scholar, and a hand search were conducted in 2023 for longitudinal studies assessing the relationship between cannabis and tobacco use in the general population. Random-effects meta-analysis was conducted to estimate odds ratios between cannabis and tobacco use in both directions. Unmeasured confounding was assessed using E-values, where a higher E-value means an unmeasured confounder would need a very strong association to explain the cannabis and tobacco relationship.

Results: From 5,387 identified records, we included 20 studies. Among cannabis-naïve individuals, baseline use of any tobacco products was positively associated with initiation of any cannabis use at follow-up (OR=5.39 [3.19,9.11]; adjusted aOR=2.59 [2.01,3.32]). In tobacco-naïve participants, baseline cannabis use was positively associated with the initiation of any tobacco use at follow-up (OR=4.08 [2.05,8.11]; aOR=2.94 [1.54,5.61]), but not significant for daily tobacco use (OR=2.95 [0.69,12.59]; aOR=2.63 [0.41,16.95]). The median E-values were 5.5 for tobacco exposure and cannabis use initiation and 4.1 for cannabis exposure and tobacco use initiation, indicating that unmeasured confounding would need a strong association with both outcomes to explain away these relationships in half of the included studies.  

Conclusion: While we found mixed evidence, more studies showed a link between cannabis and tobacco use in both directions. The common use of both cannabis and tobacco supports that ongoing public health and prevention strategies are needed to discourage uptake regardless of the sequence of use.