Aging is characterized by progressive impairment of functional capacities of all system organs, reduction in homeostatic mechanisms, and altered response to receptor stimulation. These age-related physiologic changes influence both pharmacokinetics and pharmacodynamics of drugs in elderly patients. Pharmacokinetic and pharmacodynamics changes as well as polypharmacy and comorbidities may alter significantly the effect of pharmacological treatment with advancing age. With the same drug concentration at the site of action, significant differences in the response to several drugs have been observed in older patients as compared to younger patients. Elderly patients are particularly suceptibles to the effects of frequently prescribed drugs acting on central nervous system, such as benzodiazepines, antidepressants, antipsychotics and lithium, with high potential for adverse drug reactions. Moreover, in older patients increased sensitivity to warfarin resulting in increased risk of bleeding has been previously documented. On the other hand, reduced effectiveness of conventional doses of cardiovascular drugs, such as diuretics and β-blockers, has been observed. Due to pharmacodynamic changes, therefore, dose adjustment of the above mentioned cardiovascular and psychotropic drugs is recommended in elderly. Clinicians should be aware of the age-related physiologic changes affecting several organ systems and their implications on the effect of drugs that are commonly prescribed to elderly patients.