Using drugs data in epidemiological studies is becoming fashionable with the large databases available today. We describe the philosophical basis and the main methodological issues. The process leading a patient affected by a disease to be treated with an appropriate drug appears to be complex, and influenced by external and internal many factors, described here. The disease-drug use relationship is disentangled describing its main characteristics. Drug use data means the information concerning the use of drugs, which encompasses any form of record, on paper or computerised, collected from patients, doctors prescribing the drug, pharmacists dispensing it, the drug company that produces it, or its distributor. The possible sources of drug use data are described, along with the possible strengths and weaknesses of each source. Various examples of uses of drug utilisation data as indicator in epidemiological researches are presented. Drug utilisation studies are a recent issue of public health research, merging knowledge of public health doctors, epidemiologists, pharmacologists and health economists. Automated databases demonstrate nowadays to be a valuable approach to drug utilisation investigation. In conclusion, the aim of any forward looking health care system should be to collect and freely circulate these detailed information, allowing not only useful health care planning but also indeep, population based, health research.