The World Health Organisation Patient Safety Challenge: Medication Without Harm has brought our attention to the importance of medication-related harm as a global public health issue. One of the major contributing factors is polypharmacy, the usage of multiple medicines at the same time. People are getting older and living longer with chronic diseases; they need more medications, which frequently leads to polypharmacy. Subsequently, they are at more risk of medication-related harm. The planned project is an epidemiological study on polypharmacy, medication appropriateness, risk factors, and clinical outcomes post-discharge from a hospital for internal medicine patients.

The study group hypothesise that pre- and post-admission polypharmacy and potentially inappropriate prescribing is common, especially among older patients, patients with a high comorbidity and frailty burden. Our hypothesis is additionally that preadmission polypharmacy and potentially inappropriate prescribing is associated with higher short- and long-term mortality, a longer primary hospitalization length of stay, and a higher risk of readmission.

This is an observational, retrospective, single-centered study using clinical data from the patient's medical record from the hospital, the national prescription database of the Directorate of Health, and the ICD-10 codes from primary care records. For this analysis, medicines will be classified into drug classes based on the first five characters of their WHO Anatomical Therapeutic Chemical (ATC) code. The prevalence (pre) and incidence (post) of polypharmacy (≥5 or more regular medicines) and hyper-polypharmacy (≥10 regular medicines) will be calculated. Potentially inappropriate prescribing will be assessed in individuals ≥65 years, applying Beers 2019, Start and Stopp 2014, explicit prescribing criteria. The analysis will be restricted to older individuals, as Beers criteria have not been validated in younger age groups. The anticholinergic burden will be assessed for all individuals ≥18 years will be assessed by applying the anticholinergic burden scale, and the PRIME tool will be applied to predict the likelihood of medication-related harm post-discharge. The PRIME tool consists of eight variables drawn from clinical, medication, and psychosocial domains. Applied at the point of discharge, it provides the absolute risk of an individual older adult experiencing medication-related harm during the eight weeks after discharge from acute hospital admission. This stratification is important in order to deliver targeted interventions in resource-limited healthcare settings.

Inclusion Criteria:

• Population-based cohort study that included all patients ≥18 years hospitalized in internal medicine ward at Landspitali - The National University Hospital of Iceland during the study period 1st January 2010 and 31st December 2020

Exclusion Criteria:

• Under 18 years of age

The study population is Internal medicine patients in hospital settings. The cohort will include 100% of tertiary care internal medicine admissions in Iceland and 70-80% of secondary care in Iceland. The study population includes 85942 number of participants.

There is not a plan to make IPD available.