Abstract
Background and aims
Tobacco and cannabis are commonly co‐used, and evidence for the influence of co‐use on quit outcomes for either substance is mixed. We sought to determine the efficacy of tobacco and/or cannabis use interventions, delivered to co‐users, on cannabis and tobacco use outcomes.
Method
Systematic review with meta‐analysis and narrative review, using five databases and author requests for co‐use data. Controlled and uncontrolled intervention studies focussing on treatment of tobacco and/or cannabis use assessing use of both pre and post intervention were included. Prevention interventions were excluded. Bayesian meta‐analysis was used across four outcome measures: risk ratio for tobacco and cannabis cessation post intervention separately; standardised mean change for tobacco and cannabis reduction post intervention separately. Narrative reporting of same outcome measures in non‐randomised clinical trials (non‐RCTs) and quality assessment of all included studies were conducted.
Results
Twenty studies (12 RCTs and 8 uncontrolled) were included. Bayesian meta‐analysis with informative priors based on existing data of 11 RCTs (six single substance, five multi‐substance interventions) delivered to co‐users (n= up to 1117) showed weak evidence for an effect on cannabis cessation (risk ratio [RR]=1.48 [0.92,2.49], studies=8) and no clear effect on tobacco cessation (RR= 1.10 [0.68,1.87], studies=9). Subgroup analysis suggested multi‐substance interventions might be more effective than cannabis targeted interventions on cannabis cessation (RR= 2.19 [1.10, 4.36] versus RR=1.39 [0.75,2.74]). A significant intervention effect was observed on cannabis reduction (0.25 [0.03, 0.45], studies =9) but not on tobacco reduction (0.06 [‐0.11, 0.23], studies = 9). Quality of evidence was moderate, although measurement of co‐use and of cannabis use requires standardisation. Uncontrolled studies targeting both cannabis and tobacco use indicated feasibility and acceptability.
Conclusions
Single and multi‐substance interventions addressing tobacco and/or cannabis have not shown a clear effect on either tobacco or cannabis cessation and reduction amongst co‐users. However, dual substance interventions targeting tobacco and cannabis appear feasible.