People who present frequently to emergency departments are a minority that account for disproportionate health care spending: 1 the highest 3% of this group comprise 30% of charges. 2,3 They are also high users of other health care [3][4][5][6] and are hospitalized and die more often than nonfrequent visitors to the emergency department, 7,8 suggesting a need for interventions that optimize patient outcomes and service allocation. 9 Effective interventions must recognize these patients' clinical and demographic heterogeneity. Our previous work identified 4 subgroups among patients who presented frequently to emergency departments in British Columbia, including an older subgroup with prevalent cardiac-related conditions and a younger subgroup with mental health comorbidities, 10 corroborating other studies. 11

Background:The factors that underlie persistent frequent visits to the emergency department are poorly understood. This study aimed to characterize people who visit emergency departments frequently in Ontario and Alberta, by number of years of frequent use. Methods:This was a retrospective cohort study aimed at capturing information about patients visiting emergency departments in Ontario and Alberta, Canada, from Apr. 1, 2011, to Mar. 31, 2016. We identified people 18 years or older with frequent emergency department use (top 10% of emergency department use) in fiscal year 2015/16, using the Dynamic Cohort from the Canadian Institute of Health Information. We then organized them into subgroups based on the number of years (1 to 5) in which they met the threshold for frequent use over the study period. We characterized subgroups using linked emergency department, hospitalization and mental health-related hospitalization data. Results:We identified 252 737 people in Ontario and 63 238 people in Alberta who made frequent visits to the emergency department. In Ontario and Alberta, 44.3% and 44.7%, respectively, met the threshold for frequent use in only 1 year and made 37.9% and 38.5% of visits; 6.8% and 8.2% met the threshold for frequent use over 5 years and made 11.9% and 13.2% of visits. Many characteristics followed gradients based on persistence of frequent use: as years of frequent visits increased (1 to 5 years), people had more comorbidities, homelessness, rural residence, annual emergency department visits, alcohol-and substance use-related presentations, mental health hospitalizations and instances of leaving hospital against medical advice. Interpretation: Higher levels of comorbidities, mental health issues, substance use and rural residence were seen with increasing years of frequent emergency department use. Interventions upstream and in the emergency department must address unmet needs, including services for substance use and social supports.