Evidence synthesis of medical cannabis research: current challenges and opportunities

As a wide group of medicines, the effectiveness and safety of 'medical cannabis' products is likely to vary in relation to product-specific dimensions such as potency, dosage, route of administration, and cannabinoid composition. Systematic reviews can perform a crucial role in analysing and synthesising the outcomes of medical cannabis interventions found in empirical research. We analysed 23 contemporary systematic reviews on the effectiveness and safety of medical cannabis to discern the extent to which this body of work aimed to capture, and ultimately captured, the differing outcomes of medical cannabis products by product-specific dimensions of treatment. We further highlighted the methodological reasons given by authors for an inability to describe this granular level of information. We found that a minority of systematic reviews explicitly aimed to perform a subgroup analysis to determine differences in treatment outcomes by product-specific dimensions of medical cannabis, with even fewer subsequently doing so. Authors' stated reasons for this concerned either overly large or overly small levels of variation in the characteristics, compositions, and administrations of medical cannabis products used, rendering subgroup analyses methodologically inappropriate or inapplicable. Furthering systematic reviews' abilities to capture granular information on medical cannabis treatment outcomes in relation to product-specific dimensions of treatments will require further standardisation of treatments in empirical studies.