What is ATC Classification?
The Anatomical Therapeutic Chemical (ATC) classification system is maintained by the World Health Organization Collaborating Centre for Drug Statistics Methodology. It organizes drugs into five hierarchical levels based on the organ or system they act on, their therapeutic and pharmacological properties, and their chemical characteristics.
Maintained by
WHO / WHOCC
Updated
Annually
Levels
5
Code example
C10AA05
The 5-Level Hierarchy
ATC codes are alphanumeric strings that encode all five levels. Each level adds specificity, progressing from broad anatomical classification down to individual chemical substances.
Example: C10AA05 = Atorvastatin
| Level | Code | Name |
|---|---|---|
| 1 — Anatomical | C | Cardiovascular system |
| 2 — Therapeutic | C10 | Lipid modifying agents |
| 3 — Pharmacological | C10A | Lipid modifying agents, plain |
| 4 — Chemical | C10AA | HMG CoA reductase inhibitors |
| 5 — Substance | C10AA05 | Atorvastatin |
Level 1 — Anatomical Main Groups
There are 14 main anatomical groups, each identified by a single letter.
| Code | Anatomical 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 |
| J | Antiinfectives for systemic use |
| L | Antineoplastic and immunomodulating |
| M | Musculo-skeletal system |
| N | Nervous system |
| P | Antiparasitic products |
| R | Respiratory system |
| S | Sensory organs |
| V | Various |
Why ATC Matters for Drug Analytics
Therapeutic class grouping
ATC provides a deterministic way to group drugs by class — all statins are C10AA, all ACE inhibitors are C09AA. This enables class-level spend analysis without manual curation.
Drug substitution and formulary management
Formulary analysts use ATC to identify therapeutic alternatives within the same class. Drugs at the same ATC level 4 (chemical subgroup) are typically candidates for substitution analysis.
Cross-country data harmonization
ATC is used internationally. Datasets from different countries often use ATC as the common drug classification, making cross-border utilization analysis possible.
Concurrent medication patterns
In clinical analytics and suspecting rules, ATC class prefixes are used to define concurrent medication patterns — e.g., 'patient is on another ATC C09 drug' — without enumerating individual drugs.
Sample Data — Drug-to-ATC Mapping
Below is a sample of how TwinFyRx maps drugs to their ATC classification, enabling therapeutic class grouping without manual curation.
| drug_name | atc_code |
|---|---|
| Atorvastatin 10 MG Oral Tablet | C10AA05 |
| Lisinopril 10 MG Oral Tablet | C09AA03 |
| Amlodipine 5 MG Oral Tablet | C08CA01 |
| Metformin 500 MG Oral Tablet | A10BA02 |
| Omeprazole 20 MG DR Capsule | A02BC01 |
| Duloxetine 30 MG DR Capsule | N06AX21 |
| Gabapentin 300 MG Oral Capsule | N03AX12 |
| Levothyroxine 50 MCG Tablet | H03AA01 |
| Albuterol 90 MCG Inhalation | R03AC02 |
| Apixaban 5 MG Oral Tablet | B01AF02 |
10 rows showing drug-to-ATC mappings across anatomical groups. Download a free sample.
ATC and the Defined Daily Dose (DDD)
The WHO pairs ATC classification with the Defined Daily Dose (DDD) — a technical unit of measurement representing the assumed average maintenance dose per day for a drug used for its main indication in adults. DDDs allow standardized cross-drug utilization comparisons: instead of counting tablets, you count DDD-days, which normalizes for different dose sizes and dosing frequencies. DDD is particularly useful in public health and epidemiological drug utilization research.
How TwinFyRx uses ATC
TwinFyRx includes the complete WHO ATC hierarchy in the ref.atc table, with all five levels stored individually. Every drug concept in the platform is mapped to its ATC code via the ingredient. ATC class prefixes (levels 1–4) are used in the suspecting rules engine to define concurrent medication signals — for example, “patient is on any ATC C10 drug” — without hardcoding individual drug names.