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