Introduction to Drug Utilization Research
(2003; 49 pages) View the PDF document
Table of Contents
View the documentPreface: Drug utilization research - the early work
Open this folder and view contentsChapter 1: What is drug utilization research and why is it needed?
Open this folder and view contentsChapter 2: Types of drug use information
Open this folder and view contentsChapter 3: Sources of data on drug utilization
Open this folder and view contentsChapter 4: Economic aspects of drug use (pharmacoeconomy)
Close this folderChapter 5: Drug classification systems
View the document5.1 Different classification systems
View the document5.2 The ATC classification system
View the document5.3 Ambivalence towards an international classification system
View the document5.4 Implementation of the ATC/DDD methodology
View the document5.5 General reading
View the document5.6 Exercises
Open this folder and view contentsChapter 6: Drug utilization metrics and their applications
View the documentChapter 7: Solutions to the exercises
View the documentAcknowledgements

5.2 The ATC classification system

Structure; coding principles; therapeutic use; pharmaceutical formulations; strengths

The ATC classification system divides the drugs into different groups according to the organ or system on which they act and according to their chemical, pharmacological and therapeutic properties.

Drugs are classified in groups at five different levels. The drugs are divided into 14 main groups (first level), with two therapeutic/pharmacological subgroups (second and third levels). The fourth level is a therapeutic/pharmacological/chemical subgroup and the fifth level is the chemical substance. The second, third and fourth levels are often used to identify pharmacological subgroups when these are considered to be more appropriate than therapeutic or chemical subgroups.

The complete classification of glibenclamide (see box below) illustrates the structure of the code.


Alimentary tract and metabolism (first level, main anatomical group)


Drugs used in diabetes (second level, main therapeutic group)


Oral blood-glucose-lowering drugs (third level, therapeutic/pharmacological subgroup)

A10B B

Sulfonamides, urea derivatives(fourth level, chemical/therapeutic/pharmacological subgroup)

A10B B01

Glibenclamide (fifth level, subgroup for chemical substance)

Thus, in the ATC system all plain glibenclamide preparations are given the code A10B B01.

Medicinal products are classified according to the main therapeutic use of their main active ingredient, on the basic principle of assigning 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. Two examples of this are given below:

• Sex hormones in certain dosage forms or strengths are used only in the treatment of cancer and are thus classified under L02 - Endocrine therapy. The other dosage forms and strengths are classified under G03 - Sex hormones and modulators of the genital system.

• Bromocriptine is available in different strengths. The low-dose tablets are used as prolactin inhibitors and are classified in G02 - Other gynaecologicals. Bromocriptine tablets in higher strengths are used to treat Parkinson disease and are classified in N04 - Anti-Parkinson drugs.

Different formulations with different indications may also be given separate ATC codes, for example prednisolone is given several ATC codes because of the different uses of the different formulations (see box below).

A07E A01

Intestinal anti-inflammatory agents (enemas and rectal foams)

C05A A04

Antihaemorrhoidals for topical use rectal suppositories)

D07A A03

Dermatological preparations (creams, ointments, lotions)

H02A B06

Corticosteroids for systemic use (tablets, injections)

R01A D02

Nasal decongestants (nasal spray, drops)

S01B A04

Ophthalmologicals (eye drops)

S02B A03

Otologicals (ear drops)

The ATC system is not strictly a therapeutic classification system. At all ATC levels, ATC codes can be assigned according to the pharmacological properties of the product. Subdivision on the basis of mechanism of action will understandably be rather broad, since a very detailed classification of this kind would result in having only one substance per subgroup, which is better avoided (e.g. in the case of antidepressants). Some ATC groups are subdivided into both chemical and pharmacological groups (e.g. ATC group J05A - Agents affecting the virus directly). If a new substance fits in both a chemical and pharmacological fourth level, the pharmacological group is normally chosen.

Substances classified as having the same ATC fourth level should not be considered as pharmacotherapeutically equivalent since the profiles for their mode of action, therapeutic effects, drug interactions and adverse drug reactions may differ.

As the drugs available and their uses are continuously changing and expanding, regular revisions of the ATC system are necessary. An important principle is to keep the number of alterations to a minimum. Before alterations are made, any potential difficulties arising for the users of the ATC system are considered and related to the benefits that would be achieved by the alteration. Changes to the ATC classification would be made when the main use of a drug had clearly changed, and when new groups are required to accommodate new substances or to improve the specificity of the groupings.

Because the ATC system separates drugs into groups at five levels (described above), statistics on drug utilization grouped at the five different levels can be provided. The information available ranges from figures showing total use of all drug products classified e.g. in main group C -Cardiovascular system (first level), to figures for the different subgroups (i.e. second, third and fourth level) to figures for the use of the separate substances.

More detailed information can be obtained at the lower (i.e. the fourth and fifth) levels. The higher levels are used if comparison of drug groups is the aim of a study (see Fig. 5). This gives a better overview and trends in drug use related to different therapeutic areas can easily be identified.

to previous section
to next section
The WHO Essential Medicines and Health Products Information Portal was designed and is maintained by Human Info NGO. Last updated: October 29, 2018