Bastiaan Sallevelt

11 General Introduction In 2008, two important Dutch observational studies on drug-related harm were published. A retrospective study (IPCI) found that 5% of all acute hospital admissions in adults (n = 2,238) were drug-related, which increased to almost 10% in the older population over 75 years of age [12]. In older patients, 40% of these hospitalisations were judged as potentially preventable compared to 16% in adults under 55 [12]. Similarly, the prospective Hospital Admissions Related to Medication (HARM) study concluded that 5.6% of the included 13,000 unplanned hospital admissions in adults were drug-related, of which about half were considered potentially preventable [4]. Older age, multimorbidity, polypharmacy, impaired cognition, dependent living situation, impaired renal function and non-adherence to medication regimens were identified as independent risk factors for drug-related hospital admissions [4]. These independent risk factors continue to cluster in the growing ageing population, explaining why older patients are particularly vulnerable to drug-related harm. In Europe, 20% of the total population is currently over 65 years of age, increasing to an estimated 30% by 2050 [13]. Life expectancy has risen by more than two years per decade since the 1960s. Improvements in the effectiveness of (pharmaco) therapy and healthcare coverage are key factors in these gained life-years [13– 15]. However, with ageing, the susceptibility to developing chronic diseases and multimorbidity – the co-existence of multiple chronic diseases in an individual – increases [16–18]. Multimorbidity impacts the quality of life and frequently results in polypharmacy [19,20], usually defined as the concomitant use of five or more regularly prescribed medications [21,22]. In line with ageing population’s demographic shift, polypharmacy’s prevalence has increased over the past decades (Figure 2) [23]. Figure 2. Trends in polypharmacy prevalence in older adults in the United States, Europe and New Zealand. Adopted from Wastesson et al. [23] 1