In 1996, the WHO Division of Drug Management and Policies established the WHO International Working Group for Drug Statistics Methodology. The group consists of 12 experts appointed by the WHO. The Working Group acts as expert adviser to the WHO Collaborating Centre for Drug Statistics in Oslo. New entries are considered when requests are made by manufacturers, regulatory agencies or researchers. Therefore, the system does not include substances for which no requests have been made. Changes in ATC classification are made only when the main use of a drug has clearly changed, when new groups are required to accommodate new substances or to achieve better specificity in the groupings. The DDD is also reviewed whenever an ATC code is changed. An updated list with all assignments ("ATC Index with DDDs") is issued by the Oslo Centre annually(6).