Multimorbidity and comorbidity revisited: refining the concepts for international health research

In more than 20 years since advocating for the distinction between the terms “multimorbidity” and “comorbidity” [1], there continues to be exponential growth in the amount of quantitative and qualitative research published on the prevalence, characteristics, and outcomes of living with multiple health issues in populations across the world [2–4]. As indicated in an overview of 53 systematic reviews, much of this research has relevance to patients and caregivers, academic researchers, health policy-makers, and practicing health care professionals [2].