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FAQs : The Anatomical Therapeutic Chemical (ATC) classification system
The Anatomical Therapeutic Chemical (ATC) is a classification system in which drugs are divided into different groups according to the organ or system on which they act, and their chemical, pharmacological and therapeutic properties.
Drugs are classified in groups at five different levels. The drugs are divided into fourteen main groups (1st level), with one pharmacological/therapeutic subgroup (2nd level). The 3rd and 4th levels are chemical/pharmacological/therapeutic subgroups, and the 5th level is the chemical substance. The 2nd, 3rd and 4th levels are often used to identify pharmacological subgroups when that is considered more appropriate than therapeutic or chemical subgroups.
The complete classification of metformin illustrates the structure of the code:
A Alimentary tract and metabolism (1st level, anatomical main group)
A10 Drugs used in diabetes (2nd level, therapeutic subgroup)
A10B Oral blood glucose lowering drugs (3rd level, pharmacological subgroup)
A10B A Biguanides (4th level, chemical subgroup)
A10B A02 Metformin (5th level, chemical substance)
Thus, in the ATC system all plain metformin preparations are given the ATC code A10B A02.
International non-proprietary names (INN) are preferred. If INN names are not assigned, USAN (United States Adopted Name) or BAN (British Approved Name) names are usually chosen. WHO?s list of drug terms is used when naming the different ATC levels.
The WHO Collaborating Centre for Drug Statistics Methodology in Oslo establishes new entries in the ATC classification on request from the users of the system. These include manufacturers, regulatory agencies and researchers.
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The coverage of the system is not comprehensive. A major reason why a substance is not included is that no request has been received. The Oslo Centre gives priority to preparations containing well-defined substances which have an INN name and which are:
- New Chemical Entities and biologicals proposed for licensing in a range of countries
- Existing well-defined substances used in a variety of countries
- Other medicines are considered on a case by case basis
Official ATC codes for combined preparations are assigned only to a limited extent. Combinations likely to be widely used internationally, however, are given an ATC code (e.g. beta blockers and diuretics).
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. Cross-references will be given in the guidelines to indicate the various uses of such drugs.
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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.
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 are 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 the WHO Collaborating Centre for Drug Statistics Methodology
FAQ on The Anatomical Therapeutic Chemical (ATC) classification system last updated on 11 January 2007