Drug Utilization Research (DUR) uses the Anatomical Therapeutic Chemical (ATC) as the classification system and the Defined Daily Dose (DDD) as a unit of measure. The ATC classification system groups the active medical substances according to the organ or system on which they act and according to their therapeutic, pharmacologic and chemical properties. The DDD is a unit of measurement and is linked to the ATC code. The definition of the DDD is: The assumed average maintenance dose per day for a drug used for its main indication in adults.

The ATC/DDD methodology facilitates the presentation and comparison of drug consumption statistics at international, national and regional levels despite differences in nomenclature (both branded and generic), packing sizes, pricing and customary dosages. Such methodology is useful for valid presentation and comparison of drug utilization within and across countries to support better outcomes and quality use of medicines.

In order to analyze drug use, it is essential to have both a classification system and a unit of measurement. To address the drawbacks of traditional units of measurement, a technical unit of measurement called the Defined Daily Dose (DDD) was developed for use in drug utilization studies.
International interests in the ATC/DDD methodology rapidly expanded, largely through the activity of the DURG. In 1981, the WHO Regional Office for Europe formally recognized the ATC/DDD system for drug utilization studies and recommended its use in Europe. In 1982 the WHO Collaborating Centre for Drug Statistics Methodology was established and assigned the responsibility to coordinate the development and use of the ATC/DDD methodology. In 1996, WHO recommended the global use of the ATC/DDD methodology. Several decades of experience have demonstrated its suitability in drug utilization monitoring and research. The increase in the number of users indicates the usefulness of the system.

The WHO Collaborating Centre for Drug Statistics Methodology publishes an updated version of the complete ATC index with DDDs annually and can be found here