Principles for classification

Medicinal products are classified according to the main therapeutic use of the main active ingredient, on the basic principle of only one ATC code for each pharmaceutical formulation (i.e. similar ingredients, strength and pharmaceutical form).

A medicinal product can be given more than one ATC code if it is available in two or more strengths or formulations with clearly different therapeutic uses.

A medicinal product may be used for two or more equally important indications, and the main therapeutic use of a drug may differ from one country to another. This will often give several classification alternatives. Such drugs are usually given only one code, the main indication being decided on the basis of the available literature. Problems are discussed in the WHO International Working Group for Drug Statistics Methodology where the final classification is decided. Cross-references will be given in the guidelines to indicate the various uses of such drugs.

The ATC system is not strictly a therapeutic classification system. At all ATC levels, ATC codes can be assigned according to the pharmacology of the product. Subdivision on the mechanism of action will, however, often be rather broad, since too detailed a classification according to mode of action often will result in having one substance per subgroup which as far as possible is avoided. Some ATC groups are subdivided in both chemical and pharmacological groups. If a new substance fits in both a chemical and pharmacological 4th level, the pharmacological group will normally be chosen.

Substances classified in the same ATC 4th level cannot be considered pharmacotherapeutically equivalent, since their mode of action, therapeutic effect, drug interactions and adverse drug reaction profile may differ.

Principles for changes to ATC Classification

As the drugs available and their uses are continually changing and expanding, regular revisions of the ATC system will always be necessary.

Changes in the ATC classification should be kept to a minimum. Before alterations are made, difficulties arising for the users of the ATC system are considered and related to the benefits achieved by the alteration.

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.


For more information about the ATC classification please contact WHO Collaborating Centre for Drug Statistics Methodology.