Prevalence of multimorbidity during 18 years in Denmark. A nationwide register study

<b>Background:</b> The number of patients with multimorbidity challenges healthcare systems worldwide. <b>Objective:</b> To explore the development of multimorbidity prevalence in the Danish population over the course of 18 years. <b>Design:</b> National registers were used to form eighteen cohorts including all persons aged ≥18 years; one cohort for each separate calendar year from 2000 to 2018. Multimorbidity was defined based on ten diagnosis groups (ICD-codes): lung, musculoskeletal, endocrine, mental, cancer, neurological, gastrointestinal, cardiovascular, genitourinary, and sensory organs. At least two diagnoses from different diagnosis groups had to be present to be considered multimorbidity. The diagnoses were from hospital contacts in the Danish health registers. We report both ≥2 and ≥4 multimorbidity. <b>Results:</b> From 2000-2018, the prevalence of multimorbidity increased from 7.1 % to 16.1 % with a relative increase of 126% and 314% for ≥2 and ≥ 4 diagnosis groups, respectively. The oldest age groups had the highest prevalence. In 2018, half of the population aged ≥80 years had multimorbidity. Young women showed the relatively largest prevalence increase of ≥ 4 multimorbidity. In absolute terms, the number of musculoskeletal diagnoses increased the most, whereas genitourinary, endocrinological, and musculoskeletal diagnoses showed the highest relative increase. <b>Conclusions:</b> In Denmark, the prevalence of patients with multimorbidity is increasing, and it has more than doubled from 2000-2018. The observed development in occurrence of multimorbidity over time may serve as relevant input for governments when rethinking the health care.