Guidelines for ATC classification and DDD assignment - WHOCC

Alterations in ATC classification are made when the main use of a drug has clearly
changed, and when new groups are required to accommodate new substances or to
achieve better specificity in the groupings.
When it is decided to make an alteration, the following principles are used:
- Space is provided for possible future extension of an ATC group.
- The ATC code assigned to combination products should correspond as far as
possible with the classification of the single substances in question.
- Previous ATC codes for deleted products are not used for new substances.
- Obsolete drugs or drugs withdrawn from the market are kept in the ATC system,
since exclusion of substances from the ATC system may create difficulties for
the users of the system when considering historical data.
- Changes of currently valid codes are kept to a minimum. A gap in the sequence
is preferred to changing codes.
When an ATC code is altered, the DDD is also reviewed. For example, when the
classification of chloroquine was changed from ATC group M to P (i.e. classified
only as an antimalarial), the DDD was changed since the dosages used for
treatment of malaria are different from the dosages used for rheumatic disorders.
E. The EphMRA classification system
The ATC classification system was originally based on the same main principles as
the Anatomical Classification (AC-system) developed by the European
Pharmaceutical Market Research Association (EphMRA) and the Pharmaceutical
Business Intelligence and Research Group (PBIRG).In the EphMRA system, drugs
are classified in groups at three or four different levels. The ATC classification
system is modified and extended from the EphMRA system by the addition of a
therapeutic/pharmacological/chemical subgroup as the fourth level and a fifth level
for the chemical substance.
Since 1991 there has been a consultation between the EphMRA classification
committee and the WHO Collaborating Centre for Drug Statistics Methodology in
order to achieve a better harmonisation between the two systems. The aim of this
harmonisation process is to make the systems consistent down to the ATC 3rd
level, where this is possible, and to describe the differences (i.e. show the
differences by giving bridges) and similarities in groups where harmonisation is not
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