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Changes in Healthcare Encounter Rates Possibly Related to Cannabis or Alcohol following Legalization of Recreational Marijuana in a Safety-Net Hospital

An Interrupted Time Series Analysis

Calcaterra, Susan L., MD, MPH; Hopfer, Christian J., MD; Keniston, Angela, MSPH; Hull, Madelyne L., MPH

doi: 10.1097/ADM.0000000000000480
Original Research
Editor's Choice

Objectives: Liberalization of marijuana laws in Colorado contributed to increases in cannabis-related adverse events over time. We examined characteristics of patients with healthcare encounters possibly related to cannabis and assessed the temporal association between legalization of recreational marijuana and healthcare encounters possibly attributed to cannabis.

Methods: Annual encounter rates possibly related to cannabis and alcohol were compared using negative binomial regression. Two-time intervals, pre/post-recreational marijuana legalization (January 2009 to December 2013 and January 2014 to December 2015, respectively) were used to examine changes in monthly rates of emergency encounters and hospitalizations possibly related to cannabis. Level and trend changes on encounter rates by legalization period were assessed using interrupted time series analyses. Encounters possibly related to alcohol were used as a comparator group.

Results: Most encounters identified during the study period had alcohol-related International Classification of Diseases Diagnosis and Procedural Codes (ICD-9/10-CM) codes (94.8% vs 5.2% for cannabis). Patients with encounters possibly related to cannabis were younger, more likely to be hospitalized and more likely to be admitted to the psychiatric unit than patients with encounters possibly related to alcohol. Initial and sustained effects of encounter rates possibly related to cannabis demonstrated an increased trend in slope before and after recreational marijuana legalization. The slope became more abrupt following legalization with a significant increase in trend during the post-legalization period (β = 2.7, standard error = 0.3, ρ < 0.0001). No significant change was noted for encounters possibly related to alcohol.

Conclusions: Additional research should identify patients at highest risk of an adverse health event related to cannabis and quantify costs associated with cannabis-related healthcare delivery.

Department of Medicine, Division of Hospital Medicine, Denver Health Medical Center, Denver (SLC, AK, MLH); Department of Medicine, Division of General Internal Medicine (SLC); Department of Family Medicine (SLC); Department of Psychiatry (CJH); Department of Medicine, Division of Hospital Medicine, University of Colorado Anschutz Medical Campus, Aurora, CO (AK).

Send correspondence to Susan L. Calcaterra, MD, MPH, 1216 Race Street, Denver, CO 80206. E-mail:

Received 24 April, 2018

Accepted 21 October, 2018

Disclosures: This study was funded by a divisional small grant awarded by the Division of General Internal Medicine at the University of Colorado Denver School of Medicine. The Division of General Internal Medicine had no further role in study design; in the collection, analysis and interpretation of data, in the writing of the report, or in the decision to submit the paper for publication.

Author Contribution: Dr Calcaterra drafted the original study aims, drafted the initial manuscript, and incorporated all edits for the final manuscript. Mrs Hull conducted the data analysis and drafted portions of the methods and results section. Mrs Keniston offered her expertise to assist Mrs Hull as she conducted the interrupted time series analysis and edited the final manuscript. Dr Hopfer contributed provided critical revision of important intellectual content. All authors contributed to an approved the final manuscript.

All authors report no biomedical financial interests or potential conflicts of interest.

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© 2019 American Society of Addiction Medicine