ATC code lookup and the five WHO classification levels explained
An ATC code classifies a medicine's active substance in five levels. Search the WHO index by substance name, then check its route and therapeutic use.
An ATC code identifies a position in the five-level Anatomical Therapeutic Chemical classification of medicines' active substances. Find it in the WHO Collaborating Centre's ATC/DDD Index by searching the active substance, not the brand name.
For a medicines-data or regulatory affairs task, the name match is only the start. Check the preparation's route, strength and therapeutic use before recording its code. Different routes or strengths can have separate codes when their therapeutic uses clearly differ, as the WHOCC principles explain.
What the five ATC levels mean
ATC classification groups active substances by the organ or system they act on and their therapeutic, pharmacological and chemical properties. Each level narrows the group until the fifth level identifies the chemical substance. The WHOCC's worked example uses plain metformin:
| ATC level | Code | Meaning in the metformin example |
|---|---|---|
| 1 | A | Alimentary tract and metabolism |
| 2 | A10 | Drugs used in diabetes |
| 3 | A10B | Blood glucose lowering drugs, excl. insulins |
| 4 | A10BA | Biguanides |
| 5 | A10BA02 | metformin |
Here, ATC3 means the third level: A10B. It covers a broader group than the individual substance code A10BA02. All plain metformin preparations have A10BA02, according to the same official example.
The labels do not follow one rigid pattern throughout the hierarchy. Level one is anatomical. Level two can be pharmacological or therapeutic, while levels three and four can be chemical, pharmacological or therapeutic. Calling every ATC3 group a therapeutic class would miss this distinction. Use the actual group label when describing a category in a report.
How to find the right ATC code
Use the official ATC/DDD Index, which supports searches by substance name and ATC level. All levels are searchable, so you can look up either an individual substance or a wider group.
- Identify the active substance before searching. The index does not search trademarks.
- Enter at least three letters of the substance name, or search a known ATC code.
- Read the matched entry and its higher levels to see where it sits in the classification.
- Check whether the entry fits the preparation's route, strength and therapeutic use.
- Record the index edition with your lookup so another reader can check the same reference.
The first three steps use the index's search instructions. The route, strength and use check follows the classification principles. Recording the edition is practical documentation advice, not a statutory recordkeeping requirement.
A name match can still leave more than one choice. Prednisolone, for example, has H02AB06 for systemic corticosteroids, S01BA04 for ophthalmologicals and S02BA03 for otologicals, among other codes. These WHOCC examples show why choosing the first matching substance name can give the wrong classification for a preparation.
The 2026 index lists its last update as 20 January 2026. When comparing historical data, check the classification edition used for that series rather than assuming it matches today's index.
The 14 main ATC groups
The first letter identifies the anatomical main group. The WHO ATC index lists these 14 groups:
| Code | Main group |
|---|---|
| A | Alimentary tract and metabolism |
| B | Blood and blood forming organs |
| C | Cardiovascular system |
| D | Dermatologicals |
| G | Genito urinary system and sex hormones |
| H | Systemic hormonal preparations, excl. sex hormones and insulins |
| J | Antiinfectives for systemic use |
| L | Antineoplastic and immunomodulating agents |
| M | Musculo-skeletal system |
| N | Nervous system |
| P | Antiparasitic products, insecticides and repellents |
| R | Respiratory system |
| S | Sensory organs |
| V | Various |
Use the letter for a main-group view and a more detailed code for a narrower question. For example, A and A10BA02 describe different levels of the same hierarchy. In an analysis, do not add a parent category to its own child categories: that would count overlapping categories together.
Why ATC codes appear beside DDDs
ATC and DDD support drug-utilization monitoring and research, including comparisons of consumption statistics. Classification is not a recommendation to use a medicine and does not judge its efficacy or relative efficacy.
A defined daily dose, or DDD, is the assumed average maintenance dose per day for a drug's main indication in adults. It is a standard measurement unit, not necessarily the prescribed or recommended dose. Individual doses can differ, and some medicines have no assigned DDD. See the WHOCC definition.
Reading ATC codes in Norwegian wholesale statistics
Norway's FHI uses ATC codes to organize wholesale medicine sales. Its wholesale statistics overview covers sales to pharmacies, healthcare institutions, grocery retailers, kiosks and petrol stations. Coverage includes prescription, non-prescription and veterinary medicines.
The FHI overview describes figures in total DDDs and DDDs per 1,000 inhabitants per day. The public statistics bank includes sales from 1999. Choose the ATC level that matches your question, but check the series documentation before making comparisons across years.
Wholesale sales are not a count of people taking a medicine. DDD amounts provide only a rough estimate of consumption, not an exact picture of actual use. Do not infer patient numbers, adherence or indication from an ATC-labelled wholesale total. Read the FHI coverage notes alongside the DDD limitations.
RegAffairs AI's answer about ATC codes and Norwegian drug statistics covers this lookup with source citations.