In the ATC system the drug substances are classified into groups at 5 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.
A complete classification of spironolactone with the ATC code C03DA01 illustrates the structure of the ATC system:
| C | Cardiovascular system (1st level, anatomical main group) |
| 03 | Diuretics (2nd level, therapeutic subgroup) |
| D | Potassium-sparing agents (3rd level, pharmacological subgroup) |
| A | Aldosterone antagonists (4th level, pharmacological subgroup) |
| 01 | Spironolactone (5th level, chemical substance) |
All pharmaceutical specialities containing plain spironolactone (Aldactone® and Spirix® are thus assigned the code C03DA01.
The ATC classification system makes it possible to compile drug consumption statistics on 5 different levels, that is, figures showing total consumption of all preparations classified in main group C - Cardiovascular system (1st level), figures for the various subgroups (2nd, 3rd and 4th levels), and down to figures showing consumption of each active ingredient
The ATC code for all pharmaceuticals on the Norwegian market can be found in the pharmacy medicinal product register and in the monographs of the national drug catalogue Felleskatalogen. The yellow section of the latter, entitled The Anatomical Therapeutic Chemical Medicament Register, lists all pharmaceutical products belonging to each of the ATC 5th level codes.
In 1982 the WHO Regional Office for Europe (WHO-Euro) established a coordinating centre at NMD in Oslo; the WHO Collaborating Centre for Drug Statistics Methodology. During recent years the ATC system has had a strongly increasing international dissemination. In May 1996, in order to improve the attention to this increasing global use, the Centre was attached to the WHO Headquarter in Geneva. The main tasks of the WHO Centre are to classify drug substances according to the ATC system, determine Defined Daily Doses for substances assigned an ATC code, and further develop the ATC/DDD system. This work is performed in close cooperation with a global expert group with members from 12 different countries. From January 2002 the WHO Centre has been located at the Norwegian Institute of Public Health.