Anatomical Therapeutic Chemical (ATC) classification system and the Defined Daily Dose (DDD)

The Anatomical Therapeutic Chemical (ATC) classification system and the Defined Daily Dose (DDD) as a measuring unit have become the gold standard for international drug utilization research.The ATC/DDD system is a tool for exchanging and comparing data on drug use at international, national or local levels.

The purpose of the ATC/DDD system is to serve as a tool for drug utilization 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.

A major aim of the 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. 

In 1996, when the decision on globalizing 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 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 main terms of reference of the Working Group are:

1. To provide independent evaluation of the scientific, technical, and strategic aspects of the Anatomical Therapeutic Chemical (ATC) classification system and Defined Daily Dose (DDD) methodology, including the development, implementation, and evolution of these systems within WHO and their application in global drug statistics and health data systems.
2. To recommend priorities within WHO and relevant technical units for the advancement and improvement of the ATC/DDD classification systems, ensuring they remain relevant to global public health challenges, reflect emerging therapeutic areas, and support accurate drug usage data for health policy, research, and decision-making.
3. To advise WHO on planned activities and approaches related to the further development and application of the ATC and DDD systems, including the introduction of new therapeutic categories, updates to existing classifications, and the harmonization of drug classification across international health systems and databases.
4. To advise WHO on the integration of ATC and DDD systems into broader health information systems, ensuring these classifications are effectively incorporated into global drug monitoring frameworks, health statistics, and reporting mechanisms, and supporting the work of health professionals and policymakers in assessing global drug use patterns.
5. To advise WHO on the integration of the International Nonproprietary Name (INN), Essential Medicines List (EML), and ATC/DDD systems, ensuring alignment and consistency between these systems to optimize drug classification, selection, and usage data in global health programs, enhancing access to essential medicines and supporting appropriate therapeutic use across different health systems.
6. To review and make recommendations to WHO on the effectiveness and impact of the ATC and DDD classification systems, including the consistency, accuracy, and comprehensiveness of these methodologies, and propose improvements or adjustments to enhance their global relevance, alignment with evolving therapeutic practices, and utility for global health monitoring.

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.

Meeting/open session