Effects of legal access versus illegal market cannabis on use and mental health: A randomized controlled trial

Abstract

Aims: We measured the effects of public health-oriented cannabis access compared with the illegal market on cannabis use and related mental health outcomes in adult cannabis users.

Design: This was a two-arm, parallel group, open-label, randomized controlled trial. Follow-up outcome measurement took place after 6 months.

Setting: The study was conducted in Basel-Stadt, Switzerland.

Participants: A total of 378 adult (aged ≥18 years) cannabis users were enrolled and randomized between August 2022 and March 2023, although only 374 users who completed baseline measures could be included.

Intervention and comparator: Participants were randomly assigned to the intervention group with public health-oriented recreational cannabis access in pharmacies (regulated cannabis products, safer use information, voluntary counseling, no advertisement; 189/188) or the illegal market control group (continued illicit cannabis sourcing; 189/186).

Measurements: The primary outcome was self-reported severity of cannabis misuse after 6 months, as measured by the Cannabis Use Disorders Identification Test - Revised (range 0-32). Secondary outcomes involved depressive, anxiety, and psychotic symptoms, cannabis consumption amount, alcohol, and drug use.

Findings: Ten participants were not followed (2.7%). Primary analysis included those with complete data (182 vs. 182). There was some evidence of a difference in cannabis misuse between the legal cannabis intervention group (mean [M] = 10.1) and the illegal market control group (M = 10.9; β = -0.69, 95% confidence interval [CI] = -1.4 to 0.0, P = 0.052). These results were supported by an intention-to-treat multiple imputation analysis (n = 374). Additional sub-group analysis by whether the participant used other drugs or not suggested that any reduction in cannabis misuse was confined to those in the legal cannabis intervention group who used other drugs (P_Interaction < 0.001). We found no statistically significant changes in any of the secondary outcomes.

Conclusions: Public health-oriented recreational cannabis access may decrease cannabis use and cannabis-related harms, especially among those using other drugs.

Keywords: cannabis legalization; cannabis misuse; cannabis use; mental health; randomized controlled trial; recreational cannabis.

FIGURE 1

Flow diagram from screening to 6‐months follow‐up assessment. Participants who were lost to follow‐up were those whom we were unable to contact. Other reasons included moving from the canton, dissatisfaction with cannabis quality and financial difficulties.

FIGURE 2

Change of cannabis misuse over the study period. (a) Shows cannabis misuse at baseline and at 6 months follow‐up for the total study population (a) (n = 364) and (b) stratified for participants with other drug use and those without other drug use at baseline (b) (n = 362). The interaction model was analyzed with other drug use as continuous variable. For illustrative purposes in this figure the variable was dichotomized into participants using other drugs (score > 0; n = 162) and participants not using other drugs (score = 0; n = 200) at baseline.