Many studies have suggested that ADRs are a common problem in elderly patients and are the cause of 3%-12% of hospital admissions in this age group (Williamson and Chopin, 1980; Smuck-er and Kontak, 1990; Lindley et al, 1992; Moore et al., 1998; Mannesse et al., 2000). Various risk factors have been identified. These include prescription of unnecessary or interacting drugs or drugs with relative or absolute contraindications (Lindley et al., 1992). It has also been demonstrated that ADRs are particularly likely in patients who have had a fall before admission, or in those presenting with gastrointestinal bleeding or haematuria (Mannesse et al., 2000).
Fewer studies have been done to determine the incidence of ADRs during hospital admission, but the incidence is about 5% with a range from 1.5% to over 20% (Seidl et al., 1966; Hurwitz, 1969; Skott and Geise, 1984; Leach and Roy, 1986; Lindley et al., 1992). The incidence is higher in the elderly. For example, in a prospective study of 1160 inpatients who were prescribed medication during admission, 10.2% experienced an ADR—and in patients over 60 years the incidence was higher, at 15.4% (Hurwitz, 1969). Seidl et al. (1966) found that while 13.6% of a resident hospital population in the United States acquired an ADR during hospitalisation, the incidence was as high as 24% in patients in their eighties. In addition, ADRs have been shown to be risk factors for delayed discharge from hospital (Skott and Geise, 1984) as well as early hospital readmission (Chu and Pei, 1999). Finally, in the out-patient population, about 5% -10% of patients have ADRs and slightly less than 1% of all patients are sent to hospital because of these (Skott and Geise, 1984; Chrischilles et al., 1992b).
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