Background: New approaches are required to slow down or reverse increasing trends of levels of delta-9-tetrahydrocannabinol (THC) and cannabis-attributable hospitalizations in Germany. Legal access to cannabis may constitute one viable effective policy response; however, available evidence does not suffice to inform a regulation model for Germany. The proposed study aims to reduce harm for cannabis users through legal access to herbal cannabis through pharmacies. Protocol: A quasi-experimental study comparing cannabis users with legal access to herbal cannabis (Berlin, intervention group) to those without legal access (Hamburg, control group) (total N=698). As the primary outcome, we hypothesize that: 1) illegal THC consumption will reduce by at least 50% in the intervention group and 2) total THC exposure in the intervention group will be reduced by at least 10% lower than that of the control group, taking into account baseline values. Secondary outcomes comprise measures of frequency of use, THC-impaired driving, and mode of administration. Paired t-tests and multilevel regression models will be performed for statistical analyses. Discussion: This study proposal is currently being reviewed by the 'Federal Institute for Drugs and Medical Devices' - the body responsible for approving research studies on classified substances, including cannabis. Upon approval and prior to the start of the study, a full ethical review will be undertaken. Results may inform a regulation model for Germany and other jurisdictions and are expected to deepen the understanding of the effects of legal access to cannabis. Pre-registration: German Clinical Trials Register (DRKS), DRKS00020829.
