Postal address:
WHO Collaborating Centre for Drug Statistics Methodology
Norwegian Institute of Public Health
P.O.Box 4404 Nydalen
0403 Oslo

Visiting/delivery address:
Marcus Thranes gate 6
0473 Oslo

Tel:  +47 21 07 81 60

Structure and principles

In the Anatomical Therapeutic Chemical (ATC) classification system, the active substances are divided into different groups according to the organ or system on which they act and their therapeutic, pharmacological and chemical properties. Drugs are classified in groups at five different levels.  The drugs are divided into fourteen main groups (1st level), with pharmacological/therapeutic subgroups (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 Blood glucose lowering drugs, excl. insulins
(3rd level, pharmacological subgroup)
A10BA Biguanides
(4th level, chemical subgroup)
A10BA02    metformin
(5th level, chemical substance)

Thus, in the ATC system all plain metformin preparations are given the code A10BA02.

Nomenclature (to the top)

  • 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 (Pharmacological action and therapeutic use of drugs - List of Terms) is used when naming the different ATC levels.

Inclusion and exclusion criteria (to the top)
The WHO Collaborating Centre in Oslo establishes new entries in the ATC classification on requests from the users of the system. These include manufacturers, regulatory agencies and researchers. The coverage of the system is not comprehensive. A major reason why a substance is not included is that no request has been received.

Active ingredients which fulfil one of the following criteria will normally be included in the ATC system.

  • they are new chemical entities (active ingredients) or biologicals proposed for licensing in a range of countries. A new chemical entity is normally not included in the ATC system before an application for marketing authorisation is submitted in at least one country.
  • they are existing well defined chemical entities used in a variety of countries. An INN should preferably be established for the active ingredient. Alternatively other official names, e.g. USAN or BAN names should be available.
  • they are herbal medicinal products assessed and approved by regulatory authorities based on dossiers including efficacy, safety, and quality data (e.g. the well-established use procedure in EU).

Other medicinal products are considered on a case by case basis. Complementary, homeopathic and herbal traditional medicinal products are in general not included in the ATC system.

General principles for classification (to the top)
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 route of administration (i.e. pharmaceutical forms with similar ingredients and strength will have the same ATC code).

Immediate and slow release tablets will normally have the same ATC code.

A medicinal product can be given more than one ATC code if it is available in two or more strengths or routes of administration with clearly different therapeutic uses. Two examples of this are as follows:

  • Sex hormones in certain dosage forms or strengths are only used in the treatment of cancer and are thus classified under L02 - Endocrine therapy. Remaining dosage forms/strengths are classified under G03 - Sex hormones and modulators of the genital system.
  • Clonidine is available in two different strengths.  One strength, which is used mainly in the treatment of hypertension, is classified under C02 - Antihypertensives.  Another strength is mainly used in the treatment of migraine and is classified under N02C - Antimigraine preparations.

Different pharmaceutical forms for topical and systemic use are also given separate ATC codes.

Prednisolone in single ingredient products is given several ATC codes due to different therapeutic use and different local application formulations

A07EA01 Intestinal antiinflammatory agents (enemas and foams)
C05AA04 Antihemorrhoidals for topical use (suppositories)
D07AA03 Dermatological preparations (creams, ointments and lotions)
H02AB06 Corticosteroids for systemic use (tablets, injections)
R01AD02 Nasal decongestants (nasal sprays/drops)
S01BA04  Ophthalmologicals (eye drops)
S02BA03  Otologicals (ear drops)

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 only given 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 a too detailed classification according to mode of action often will result in having one substance per subgroup which as far as possible is avoided (e.g. antidepressants).  Some ATC groups are subdivided in both chemical and pharmacological groups (e.g. ATC group J05A - Direct acting antivirals).  If a new substance fits in both a chemical and pharmacological 4th level, the pharmacological group should 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.

Normally, different stereoisomeric forms will have separate ATC codes.  Exceptions will be described in the guidelines for the respective ATC groups.

A new medicinal substance not clearly belonging to any existing ATC 4th level group of related substances will as a main rule be placed in an X group ("other" group).  To avoid a situation of several 4th levels with only one single substance in each, new 4th levels are as a general rule only established when at least two substances with marketing authorisations fit in the group. In addition, a new 4th level should be regarded a benefit for drug utilization research. New and innovative medicinal products will therefore often be classified in an X group and such groups could be established for only one single substance.

Prodrugs are usually assigned separate ATC codes if the dosages used are different and/or the nonproprietary name of the prodrug and the active drugs are different.

J01CA08  pivmecillinam
J01CA11  mecillinam

Classification of plain products (to the top)
Plain products are defined as:

  • Preparations containing one active component (including stereoisomeric mixtures).  
  • Medicinal products which in addition to one active component contain auxiliary substances intended to increase the stability of the preparations (e.g. vaccines containing small amounts of antibacterials), increase the duration (e.g. depot formulations) and/or increase the absorption (e.g. different solvents in various dermatologicals) are also considered as plain products.

Plain products are classified according to the general principles outlined above.

Classification of combination products (to the top)
Products containing two or more active ingredients are regarded as combination products.  Combination products are classified according to three main principles.


Combination products containing two or more active ingredients belonging to the same 4th level are normally classified using the 5th level codes 20 or 30.

N01BB02 lidocaine
N01BB04 prilocaine
N01BB20 combinations (e.g. lidocaine and prilocaine)

b) Combination products containing two or more active ingredients not belonging to the same 4th level are classified using the 50-series.

R06AA02 diphenhydramine
R06AA52 diphenhydramine, combinations

Different combination products sharing the same main active ingredient are usually given the same ATC code.  Thus, products containing phenylpropanolamine + brompheniramine and phenylpropanolamine + cinnarizine are both given the code R01BA51.

c) Combination products containing psycholeptic drugs, which are not classified under N05 - Psycholeptics or N06 - Psychoanaleptics, are classified at separate 5th levels using the 70-series.

N02BA71 - acetylsalicylic acid, combinations with psycholeptics (products containing other substances in addition to a psycholeptic are also classified here.)

Some of the combination products containing psycholeptics have been classified at a separate third or fourth level (e.g. A03C - Antispasmodics in combination with psycholeptics).

There are some exceptions to the main rules and these are explained in the guidelines.

Separate ATC 3rd or 4th levels have been assigned for some important combinations, e.g. beta blocking agents and diuretics.

It may be difficult to decide where a certain combination should be classified.  The main therapeutic use decides the classification.  A medicinal product containing an analgesic and a tranquillizer, and used primarily to ease pain, should be classified as an analgesic.  Likewise, a preparation used for functional gastrointestinal disorders will be classified under A03, Drugs for functional gastrointestinal disorders, even if it contains small amounts of analgesics and/or psycholeptics.  Similar examples are described in detail in the guidelines for the relevant drug groups.

In some ATC groups a ranking is introduced to help in the classification of combination products (e.g. combinations of different antihypertensives and combinations of different analgesics).  This ranking shows which drug takes precedence over others when the classification is decided.  This is detailed in the guidelines for the relevant drug groups.

(to the top)

Last updated: 2011-03-25