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Background: Drug use increases with advancing age, and in older patients it is associated with an increase in adverse drug reactions (ADRs). ADRs are a primary cause of morbidity and mortality worldwide.Objectives: To evaluate the prevalence, clinical characteristics and avoidability of ADR-related hospital admissions in elderly patients.Methods: From November 2004 to December 2005, all patients aged >or=65 years consecutively admitted to the Geriatric Unit of the Casa Sollievo della Sofferenza Hospital, San Giovanni Rotondo in Italy, were evaluated for enrolment in the study. ADRs were defined according to the WHO Adverse Reaction Terminology system. Drugs were classified according to Anatomical Therapeutic Chemical classification system. The Naranjo algorithm was used to evaluate the relationship between drug use and the ADR (definite, probable, possible or doubtful) and Hallas criteria were used to evaluate the avoidability of the ADR (definitely avoidable, possibly avoidable or unavoidable). All cases of a suspected ADR were discussed by a team trained in drug safety, including three geriatricians, one clinical pharmacologist and one pharmacist. Only cases of an ADR with an agreement >or=80% were included.Results: Of the 1756 patients observed, 102 (5.8%, 42 males, 60 females, mean age 76.5 +/- 7.4 years, range 65-93 years) showed certain (6.8%) or probable (91.2%) ADR-related hospitalization. Gastrointestinal disorders (48 patients, 47.1%); platelet, bleeding and clotting disorders (20 patients, 19.6%); and cardiovascular disorders (13 patients, 12.7%) were the most frequent ADRs. NSAIDs (23.5%), oral anticoagulants (20.6%), low-dose aspirin (acetylsalicylic acid) [13.7%] and digoxin (12.7%) were the drugs most frequently involved in ADRs. Of the ADRs, 45.1% were defined as definitely avoidable, 31.4% as possibly avoidable, 18.6% as unavoidable and 4.9% as unclassifiable. Of 78 patients with definitely or possibly avoidable ADRs, 17 patients (21.8%) had received an inappropriate prescription, 29 patients (37.2%) had not received a prescription for an effective gastroprotective drug concomitantly with NSAID or low-dose aspirin treatment and 32 patients (41%) were not monitored during drug treatment.Conclusion: In the elderly, almost 6% of hospitalizations are ADR related. Most of these ADRs are potentially avoidable. Strategies that reduce inappropriate prescriptions and monitoring errors, as well as improving active prevention of ADRs, are needed in elderly subjects. [ABSTRACT FROM AUTHOR]