In Denmark, there are plans for establishing a national decision support system, providing on-line support for physicians during drug prescribing. This includes establishment of a national database containing information about each patient's drug allergies. Allergy information already exists in medication modules in hospital systems and primary care systems and thus constitutes a potential source for the national allergy database. This paper reports an analysis of local data structure, content and registration policies with the aim to re-use existing allergy data. The result of the analysis is that due to lack of harmonisation most existing cannot be re-used in the national database. The paper propose a common dataset for allergy data where national and international standards were considered.