The ATC/DDD Methodology

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.

The methodology is endorsed by WHO and is recommended as the international standard for drug utilization monitoring and research. The adoption of ATC/DDD by more users further facilitates the comparison of data at an international level.

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How did ATC/DDD start?

The field of Drug Utilization Research (DUR) began attracting attention in the 1960’s. This followed the publication of a breakthrough study on drug consumption from 1966-1967 (pioneered by the WHO Regional Office for Europe) which further exemplified the importance and applicability of DUR (1). In addition, the WHO symposium in 1969 highlighted the need for an internationally accepted classification system for drug utilization studies. As a result the Drug Utilization Research Group (DURG) was established and entrusted with the development of DUR methods. Inspired by this interest, the Anatomical Therapeutic Chemical (ATC) classification was developed in Norway as a modification and extension of the  European Pharmaceutical Market Research Association (EPhMRA) classification system

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.


1Engel A, Siderius P. The consumption of drugs. Report on a study, 1966-1967. WHO Regional Office for Europe, Copenhagen 1968 (EURO 3101).

Responsible Organizations of ATC/DDD methodology

Who is who?

The World Health Organization: The WHO endorsed the ATC/DDD methodology for global use in 1996 as an international standard for drug utilization studies in an effort to ensure universal access to essential drugs and to stimulate rational use of drugs particularly in middle/low income countries.

The WHO Collaborating Centre for Drug Statistics Methodology : The central body responsible for the development and maintenance of the ATC/DDD system. This Centre is located at the Norwegian Institute of Public Health (NIPH) and the main activities of the Centre are drawn up in an agreement between the WHO Headquarters and the Government of Norway. These activities are:

  • To classify drugs according to the ATC system.
  • To establish DDDs for drugs which have been assigned an ATC code.
  • To review and revise as necessary the ATC classification system and DDDs.
  • To stimulate and influence the practical use of the ATC system by cooperating with researchers in the drug utilization field.
  • To organize training courses in the ATC/DDD methodology and to lecture such courses and seminars organized by others.
  • To provide technical support to countries in setting up their national medicines classification systems and build capacity in the use of medicines consumption information.

WHO International Working Group for Drug Statistics Methodology : This body includes 12 expert members selected by WHO Headquarters, who represent a wide range of geographical and professional backgrounds. All six WHO regions are represented in the group. This body gives expert advice to the WHO Collaborating Centre. The main terms of reference of the Working Group are:

  • To continue the scientific development of the ATC/DDD system.
  • To discuss and approve all new ATC codes, DDD assignments and alterations to existing ATC codes and DDDs.
  • To develop further the use of the ATC/DDD system as an international standard for drug utilization studies.
  • To revise as necessary the guidelines for assignment and change of ATC codes and DDDs.
  • To revise as necessary the procedures for applications for assignment of and changes to ATC codes and DDDs to ensure they are consistent and transparent.
  • To assess the sources and availability of statistics on drug use internationally, and to encourage the systematic collection of comprehensive drug use statistics in all countries and regions using the ATC/DDD system as the international standard.
  • To develop methods, manuals and guidelines for the practical application and appropriate use of the ATC/DDD system in drug utilization studies in a variety of settings, particularly those applicable to developing countries.
  • To work with groups involved in rational drug use initiatives to integrate methods for measurement of drug use in assessing needs and outcomes of interventions with the aim of improving drug use.

The International Working Group meets twice annually, and the WHO Collaborating Centre for Drug Statistics Methodology acts as secretariat for the group.

Guidance Material

Guidelines

The ATC/DDD guidelines and the Index were both published for the first time in the current format in 1990. The system has been in use for statistics since 1975. Both publications are updated annually and available electronically and in hard copies. English and Spanish versions of the publications are available.

Guidelines for ATC Classification and DDD assignment

A comprehensive guide regarding ATC and DDD can be found  here . The Guidelines includes information on the ATC Classification system and general principles of DDDs assignment. Information necessary for allocating ATC codes and DDDs at a national or local level can be found in the Guidelines.

The ATC index with DDDs

The index includes all officially assigned ATC codes and DDDs at the time of issuing. Changes to the index are made annually and a cumulative list including all ATC and DDD alterations made since 1982 is available  here .