The ATC classification system: how medicines are grouped worldwide

The Anatomical Therapeutic Chemical (ATC) classification system is a globally recognized framework used to group medicines according to the organs or systems they affect and their therapeutic, pharmacological, and chemical properties. Developed in the 1970s as a tool for drug utilization research, the system allows health authorities, researchers, and regulators to track medicine use and compare trends across different countries over time. Rather than organizing medicines by brand name, commercial origin, or manufacturer, the ATC system focuses entirely on the active ingredients themselves. This provides a common international language for describing medical substances, ensuring that data sharing across borders remains accurate and consistent.

The Five Levels of an ATC Code

Every ATC code is structured into five distinct levels, creating a hierarchical code that moves from broad anatomical categories down to specific chemical compounds. The first level is an anatomical main group, represented by a single Roman capital letter. For example, 'A' denotes the alimentary tract and metabolism, while 'N' represents the nervous system. The second level adds two numbers to indicate the therapeutic main group, such as '02' for analgesics, giving a code like N02. The third level adds a letter to define the pharmacological subgroup, and the fourth level adds another letter for the chemical subgroup. Finally, the fifth level specifies the exact chemical substance using a two-digit code. For instance, the complete ATC code for metformin is A10BA02: it acts on the alimentary tract (A), is used for diabetes (10), is a blood glucose-lowering drug excluding insulins (B), belongs to the biguanide subgroup (A), and is specifically metformin (02).

WHO Maintenance and Defined Daily Doses

The ATC classification is maintained by the World Health Organization (WHO) Collaborating Centre for Drug Statistics Methodology, located in Oslo, Norway. This centre is responsible for assigning new codes, updating existing classifications as new drugs emerge, and managing the accompanying Defined Daily Dose (DDD) system. The DDD is a statistical measure of the assumed average maintenance dose per day for a drug used for its main indication in adults. While the DDD is not a recommended prescription dose, it serves as a technical unit of measurement that helps researchers standardise and compare aggregate medicine consumption across different regions. Because the system is managed internationally, it provides a neutral, scientific standard that is independent of local market dynamics or commercial branding.

How ATC Differs From Pack-Level Codes

While an ATC code describes the active ingredient and its therapeutic action, it does not identify a specific pharmaceutical product, strength, or pack. This is where national pack-level codes become necessary. In the United States, the National Drug Code (NDC) is used to uniquely identify the specific manufacturer, formulation, dosage, and packaging size of a medication. Similarly, South Africa uses the National Pharmaceutical Product Interface (NAPPI) code to identify exact products for pharmacy dispensing, stock control, and medical aid claims. A single ATC code can apply to dozens of different products, brands, and pack sizes, whereas NAPPI and NDC codes are strictly unique to an individual pack. For example, every registered 500mg pack of metformin tablets will share the same ATC code, but each will have its own unique NAPPI or NDC code based on the manufacturer and pack size.

Searching for Medicines Across Systems

The distinction between ingredient-level classifications and pack-level codes is essential for navigating pharmaceutical databases accurately. When searching for a medication, an active ingredient name or ATC code will provide a broad view of all available options, while a pack-level code will point to a specific, purchasable item. In South Africa, tools like HealthSA bridge this gap by linking the active ingredient and ATC grouping to specific pack-level NAPPI codes and the Single Exit Price, helping users find exact products efficiently without getting lost in overlapping brand names. Ultimately, the ATC system provides the organizational backbone that makes global medicine tracking, drug utilization research, and local price comparison possible, ensuring that a substance is categorized consistently regardless of where it is sold.