ultimorbidity, the co-occurrence of multiple chronic illnesses, has increased in prevalence as populations age and improved medical treatment extends the lives of patients with chronic illness. 1,2 Mental disorders affect as many as 30% of people with multimorbidity. 1 Mental disorders are also associated with more severe physical illness, including increased symptom burden, shortened lifespan and compounded disability. 3,4 Both physical multimorbidity and mental disorders are growing individual and public-health burdens, associated with disability, mortality and use of primary care services. 1,3,5,6 Treatment for patients with multimorbidity is complex and may be less effective if care for each condition is planned in isolation. 1,2 Gaps in care may lead to potentially preventable visits to the emergency department to treat exacerbations or complications of chronic illness, including mental disorders. 7-16 Although the independent effects of physical illnesses and mental disorders on frequent emergency department visits are known, their combined effect (also called synergy or interaction) has not been well studied. However, such synergy has been observed with work-related disability, for which people with mental and physical disorders have a disproportionately greater risk. 6 The RESEARCH MENTAL HEALTH

Background Frequent healthcare users place a significant burden on health systems. Factors such as multimorbidity and low socioeconomic status have been associated with high use of ambulatory care services (emergency rooms, general practitioners and specialist physicians). However, the combined effect of these two factors remains poorly understood. Our goal was to determine whether the risk of being a frequent user of ambulatory care is influenced by an interaction between multimorbidity and socioeconomic status, in an entire population covered by a universal health system. Methods Using a linkage of administrative databases, we conducted a population-based cohort study of all adults in Quebec, Canada. Multimorbidity (defined as the number of different diseases) was assessed over a two-year period from April 1st 2012 to March 31st 2014 and socioeconomic status was estimated using a validated material deprivation index. Frequents users for a particular category of ambulatory services had a number of visits among the highest 5% in the total population during the 2014–15 fiscal year. We used ajusted logistic regressions to model the association between frequent use of health services and multimorbidity, depending on socioeconomic status. Results Frequent users (5.1% of the population) were responsible for 25.2% of all ambulatory care visits. The lower the socioeconomic status, the higher the burden of chronic diseases, and the more frequent the visits to emergency departments and general practitioners. Socioeconomic status modified the association between multimorbidity and frequent visits to specialist physicians: those with low socioeconomic status visited specialist physicians less often. The difference in adjusted proportions of frequent use between the most deprived and the least deprived individuals varied from 0.1% for those without any chronic disease to 5.1% for those with four or more chronic diseases. No such differences in proportions were observed for frequent visits to an emergency room or frequent visits to a general practitioner. Conclusion Even in a universal healthcare system, the gap between socioeconomic groups widens as a function of multimorbidity with regard to visits to the specialist physicians. Further studies are needed to better understand the differential use of specialized care by the most deprived individuals.