ATC
Overview
The Anatomical Therapeutic Chemical (ATC) Classification System is the WHO-endorsed international standard for classifying drug substances by the organ or system they act on, and by their therapeutic, pharmacological, and chemical properties. Developed in Norway in the 1970s, it was formalised by the WHO Collaborating Centre for Drug Statistics Methodology (established 1982, hosted at the Norwegian Institute of Public Health in Oslo) and recommended for global use by WHO in 1996. ATC is paired with the Defined Daily Dose (DDD) — a unit of measurement representing the assumed average adult maintenance dose per day — to form the ATC/DDD methodology, the international standard for drug utilisation research and pharmacoepidemiology.
Structure
ATC codes have a five-level hierarchy:
| Level | Type | Example |
|---|---|---|
| 1st | Anatomical main group (letter) | N (Nervous System) |
| 2nd | Therapeutic subgroup (2 digits) | N05 (Psycholeptics) |
| 3rd | Therapeutic/pharmacological subgroup (letter) | N05A (Antipsychotics) |
| 4th | Chemical/pharmacological subgroup (letter) | N05AH (Diazepines, oxazepines…) |
| 5th | Chemical substance (2 digits) | N05AH03 (Olanzapine) |
There are 14 anatomical main groups (A–V). The index is updated annually each January 1st. A substance can have multiple ATC codes for different routes of administration or indications.
ATC and DDD in Drug Utilisation Research
ATC is a standardised vocabulary in the OMOP CDM Athena vocabulary browser, where it supplements RxNorm as a drug classification hierarchy. It is used for drug utilisation studies and pharmacoepidemiology, imputing drug exposure duration via DDD-based calculations, and mapping across different national drug coding systems.
Resources
- ATC classification: https://www.who.int/tools/atc-ddd-toolkit/atc-classification
- ATC/DDD index (searchable): https://atcddd.fhi.no/atc_ddd_index/
- ATC/DDD methodology: https://www.who.int/tools/atc-ddd-toolkit/methodology
- Athena OMOP vocabularies: https://athena.ohdsi.org