The overall aim of this thesis is to investigate drug use in institutionalized and home- dwelling elderly, with a special focus on dementia. The major findings from the separate studies are summarized below. Study I: In this register-based study of 1 260 843 home-dwelling and 86 721 institutionalized persons aged ≥65 years, 30% of the institutionalized and 12% of the home-dwelling elderly were exposed to potentially inappropriate drug use (PIDU). Institutionalization was associated with overall PIDU (OR 2.36, 95% CI 2.29-2.44), after adjustment for age, sex and number of drugs (i.e. a proxy for overall co- morbidity). Study II: We studied the use of analgesics and psychotropics in 2 610 persons aged ≥66 years who participated in the baseline examination in the Swedish National Study on Aging and Care-Kungsholmen (SNAC-K). About 46% of the persons with dementia and 25% of those without dementia used analgesics. Also, 63% of the persons with dementia compared to 32% of those without dementia used psychotropics. The prevalence of pain-related diagnoses was similar in persons with and without dementia. However, having a pain-related diagnosis was associated with use of psychotropics in persons with dementia, but not in those without dementia. Study III: In this study based on data from SNAC-K and the National Patient Register, persons with dementia had a higher prevalence of osteoporotic fractures in the previous four years than persons without dementia (i.e. 25% compared to 7%). Persons with dementia were, however, less likely to use osteoporosis drugs (OR 0.34, 95% CI 0.19- 0.59), after controlling for age, sex, osteoporotic fractures and type of housing (own home or institution). Study IV: In this nationwide register-based study, we analyzed use of antibiotics commonly used to treat lower urinary tract infection (UTI). We found that use of trimethoprim in institutionalized women was more than twice as common as the recommended level. Also, the use of quinolones, in women treated with UTI antibiotics, was high in ...