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About 40% of the intoxications after drug administration occur in the elderly. A significant proportion of the disease states in elderly patients is related to adverse reactions to prescribed drugs. Declining renal function, a reduction in both renal blood flow and glomerular filtration rate, is a major contributor to drug toxicity in the elderly. Therefore, a review (based on newer papers from Medline) of age-dependent changes of the kidneys and their consequences for drug therapy in geriatric patients is presented. Renal changes that occur with aging are: a decrease of renal weight, a thickening of the intrarenal vascular intima, sclerogenous changes of the glomeruli, and infiltration of chronic inflammatory cells and fibrosis in the stroma. Altered renal tubular function, including impaired handling of water, sodium, acid, and glucose, is also frequently present in old age. Impaired 'endocrinologic' functioning manifested by changes of the renin-angiotensin system, vitamin D metabolism, and antidiuretic hormone responsiveness has been reported. The aging kidney is constantly exposed to the effects of a variety of potential toxic processes, i.e., drugs and chronic illnesses including hypertension, diabetes, and atherosclerotic disease. Renal changes that occur with aging also consist of impairment in the ability to concentrate urine and to conserve sodium and water. These physiological changes increase the risks of volume depletion and prerenal type of acute renal failure. A frequent cause of acute renal failure in the elderly is drug-induced nephropathy. Nonsteroidal anti-inflammatory drugs, antibiotics, and diuretics are most often involved. Due to the age-dependent decline of renal function, the pharmacokinetics of many drugs are altered in elderly patients. Therefore, the most important renal function to monitor with aging is the creatinine clearance. Changes in pharmacokinetics of many drugs and most decisions on drug dosage can be based on this information alone, as tubular functions of the kidney decrease at rates paralleling the age-dependent decrease in glomerular filtration rate (which is approximately measured by the creatinine clearance). As a conclusion, age-dependent changes of renal function are not only responsible for changes in pharmacokinetics and pharmacodynamics. In many cases, the kidneys are the target organ of adverse drug reactions too.