ATC/DDD methodology

Published 11.03.2025

The purpose of the ATC/DDD system is to serve as a tool for drug utilization monitoring and research in order to improve quality of drug use. One component of this is the presentation and comparison of drug consumption statistics at international and other levels.

Purpose of the ATC/DDD System

The purpose of the ATC/DDD system is to serve as a tool for drug utilization monitoring and research in order to improve the quality of drug use. One component of this is the presentation and comparison of drug consumption statistics at international and other levels.

A major aim of the ATC/DDD Centre and Working Group is to maintain stable ATC codes and DDDs over time to allow trends in drug consumption to be studied without the complication of frequent changes to the system. There is a strong reluctance to make changes to classifications or DDDs where such changes are requested for reasons not directly related to drug consumption studies. For this reason the ATC/DDD system by itself is not suitable for guiding decisions about reimbursement, pricing and therapeutic substitution.

It is essential that a tool for drug utilization monitoring and research is able to cover most medicines available on the market. An important aim of drug utilization is to monitor rational as well as irrational drug use as an important step in improving the quality of drug use.

The classification of a substance in the ATC/DDD system is therefore not a recommendation for use and it does not imply any judgements about efficacy or relative efficacy of drugs and groups of drugs.

History

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 (ref: Engel A, Siderius P. The consumption of drugs. Report on a study, 1966-1967. WHO Regional Office for Europe, Copenhagen 1968 (EURO 3101). 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 internationally applicable methods for DUR. 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 measure drug use, it is important to have both a classification system and a unit of measurement. To deal with the objections against traditional units of measurement, a technical unit of measurement called the Defined Daily Dose (DDD) was developed for use in drug utilization studies.

Several decades of experience using ATC/DDD methodology have demonstrated its suitability in drug utilization monitoring and research. The increase in number of users indicates the usefulness of the system.

Access to standardised and validated information on drug use is essential to allow audit of patterns of drug utilization, identification of problems, educational or other interventions and monitoring of the outcomes of the interventions. In 1981, the ATC/DDD system was recommended by the WHO as the international standard for drug utilization studies, and in 1982 the WHO Collaborating Centre for Drug Statistics Methodology (the ATC/DDD Centre) was established and given the responsibility for coordinating the development and use of the ATC/DDD system. In 1996, the ATC/DDD Centre was recognized as a global centre. This was seen as important to allow close integration of international drug utilization studies and WHO’s initiatives to achieve universal access to needed drugs and rational use of drugs particularly in developing countries. Access to standardised and validated information on drug use is essential to allow audit of patterns of drug utilization, identification of problems, educational or other interventions and monitoring of the outcomes of the interventions.

When the decision on globalizing the ATC/DDD system was taken, the WHO Headquarter established the WHO International Working Group for Drug Statistics Methodology. The WHO Collaborating Centre for Drug Statistics Methodology receives expert advice from the Working Group.

WHO Collaborating Centre for Drug Statistics Methodology

The ATC/DDD Centre was established in 1982. The ATC/DDD Centre is located in Oslo at the Norwegian Institute of Public Health and funded by the Norwegian government.

The ATC/DDD Centre's main activities are development and maintenance of the ATC/DDD system, including:

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 co-operating 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

In 1996, when the decision to globalise the ATC/DDD system was taken, the WHO Division of Drug Management and Policies established the WHO International Working Group for Drug Statistics Methodology. The International Working Group includes 12 expert members selected by WHO Headquarters to represent a wide range of geographical and professional backgrounds, including clinical pharmacology, clinical medicine, international public health, drug utilization and drug regulation. All six WHO regions are represented in the group. The WHO Collaborating Centre for Drug Statistics Methodology receives expert advice from the Working Group.

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. A teleconference may replace one of the two annual meetings. Members are required to complete a WHO declaration of interest form before the meeting. Observers from the WHO Collaborating Centre for International Drug Monitoring and the International Federation of Pharmaceutical Manufacturers Association are invited to attend the meetings of the International Working Group.

An open session is arranged prior to one of the annual meetings to which any interested party can register.

Decisions on ATC classification or DDD assignment from the meetings are published on the website of the WHO Collaborating Centre for Drug Statistics Methodology and in the publication WHO Drug Information. Any decision on a new or revised ATC classification or DDD assignment is first published as temporary. Any interested party wishing to dispute this decision is invited to comment within a specified deadline after its publication. If there are no objections to a temporary decision, it will be published as a final decision and implemented in the next issue of the ATC classification index with DDDs. In case of any objection, the decision will be reconsidered at the next meeting of the International Working Group. If a new decision is made at the second meeting, this decision will be published as temporary and will be open to comments similar to the first decision. WHO has the final responsibility for the decisions and any dispute arising in the course of this work must be referred to WHO for final resolution.

The EphMRA classification system

The ATC classification system was originally based on the same main principles as the Anatomical Classification developed by the European Pharmaceutical Market Research Association (EphMRA) and the Pharmaceutical Business Intelligence and Research Group (PBIRG).

In the EphMRA classification, drugs are classified in a hierarchy of three and sometimes four levels mainly according to their indications and use. Many of these quite similar to the ATC structure, but in many groups, less detailed. There are no specific codes for the active ingredient (5th level in the ATC). Contrary to the ATC classification, EphMRA classifies medicinal products. It is possible to find products with the same active ingredient, route of administration and strength in several classes. Despite a similar structure at the higher levels, the ATC classification and the EphMRA classification have developed individually for many years.

Since 1991 there has been an annual consultation between the EphMRA classification committee and the WHO Collaborating Centre for Drug Statistics Methodology to discuss classification problems and to harmonize when possible. The harmonization process was initiated in order to minimise the confusion of having two very similar classification systems.

There are many differences between the EphMRA classification and the ATC classification. This means that data prepared using the ATC classification cannot be directly compared with data prepared using the EphMRA system. Awareness of the differences between the two systems is then particularly important. In some settings, and on the EphMRA website, the system is referred to as the ATC classification and this has caused confusion among users over the years.

The EphMRA classification system is used worldwide by IQVIA (IMS Health/Quintiles) in producing marketing research statistics for the pharmaceutical industry.

An annually updated comparison booklet of the two systems is available.