The application of multiple medications has successively increased during a number of years and has thereby increased the potential risks of adverse drug reactions, interactions and non-adherence to drug therapy. This may result in unnecessary health expenditure, directly due to redundant drug sales, and indirectly due to the increased hospitalization caused by drug-related problems. The overall aim of this thesis was to investigate the occurrence and development of drug use, multiple medication, and polypharmacy in an entire national population by using individual-based data on dispensed drugs. The studies (I-V) in the thesis are based on data of dispensed prescription drugs for up to 6.2 million individuals obtained from the Swedish Prescribed Drug Register. The data in the studies cover different periods of time between July 2005-Sept 2008, and the data have been analyzed on the basis of epidemiological measures and statistical methods. The major conclusions of the studies are: the prevalence of dispensed drugs and multiple medications was extensive in all age groups and was higher for females than for males. Multiple medications should be regarded as a risk in terms of potential drug-drug interactions and adverse drug reactions in all age groups (I). Regional differences in the prevalence of polypharmacy were observed and partly explained by the regional age distribution in Sweden. The use of a novel weighted polypharmacy index indicated regional differences in drug therapy for individuals with polypharmacy (II). The number of drugs used by an individual not only increased the potential risks associated with multiple drug use, but also the potential burden of an increased therapeutic intensity, especially for elderly (III). Individuals with ten or more drugs accounted for almost fifty percent of the total acquisition costs of dispensed drugs. Therefore, interventions with a focus on the reduction of the number of prescription drugs for the small group of patients with a large number of different drugs may also result in a substantial reduction in the total drug costs (IV). In spite of national and regional efforts to reduce polypharmacy, the prevalence of polypharmacy and excessive polypharmacy and the mean number of drugs per individual continued to increase in Sweden 2005-2008 (V). The observed year-by-year increase in polypharmacy underlines the importance of monitoring the development of drug use in all ages. Individual-based registers studies of dispensed drugs provide high quality data and could serve as the basis for further research and also in terms of training health care personnel. It can also be used as a base for interventions and the evaluation of drug use. To enable better comparisons on drug use and its consequences, there is a need for standards concerning measurements, classification and criteria which encompass all types of medications. For clinicians, there is a need for information concerning the patient’s actual use of all different types of medications.