The 'grass ceiling': Limitations in the literature hinder our understanding of cannabis use and its consequences

Aim: To illustrate how limitations in the cannabis literature undermine our ability to understand cannabis-related harms and problems experienced by users and identify users at increased risk of experiencing adverse outcomes of use. Method and results: Limitations have been organized into three overarching themes. The first relates to the classification systems employed by researchers to categorize cannabis users, their cannabis use and the assumptions on which these systems are based. The second theme encompasses methodological and reporting issues, including differences between studies, inadequate statistical control of potential confounders, the under-reporting of effect sizes and the lack of consideration of clinical significance. The final theme covers differing approaches to studying cannabis use, including recruitment methods. Limitations related to the nature of the data collected by researchers are discussed throughout, with a focus on how they affect our understanding of cannabis use and users. Conclusions: These limitations must be addressed to facilitate the development of effective and appropriately targeted evidence-based public health campaigns, treatment programmes and preventative, early intervention and harm minimization strategies, and to inform cannabis-related policy and legislation.