Conducting pharmacoepidemiologic research in Denmark.

Denmark has one of the worlds most comprehensive registration of its citizens' use of medical and social services. Most databases are population-based and of very high quality. Virtually all employ a mutual person identifier, which renders it technically possible to link any of them with others. There are two prescription registries of interest for research, the OPED and the NJPD, each covering 0.5 million persons. The content of these are described in brief. The most recent Danish data protection act can be viewed as a liberalization of prevailing registry practice. Our most important obstacles for performing record-linkage studies are costs, academic resources and a lack of generally accepted guidelines on the ethics of observational research.