Pharmaceutical expenses are 19% of social security expenses in Spain. Studies of prescription habits, prescription prevalence, therapeutic profiles, prescription compliance and incidence reports are thus required. Different methods to study utilization of drugs are discussed and examples of each method are provided. The number of registered pharamceutical specialities varies between 21000 in Germany (2900 active substances) and 1000 (700) in Iceland. The thiazide consumption varies between 4.3 daily defined doses (DDD) per 1000 inhab./day in Spain and 42.9 in Sweden. Medication errors were caused by recording problems in 6.5% of all drug distributions.