Anatomical Therapeutic Chemical Classification System

The Anatomical Therapeutic Chemical (ATC) Classification System is used for the classification of drugs. It is controlled by the WHO Collaborating Centre for Drug Statistics Methodology (WHOCC), and was first published in 1976.

The classification system divides drugs into different groups according to the organ or system on which they act and/or their therapeutic and chemical characteristics. Each bottom-level ATC code stands for a pharmaceutically used substance in a single indication (or use). This means that one drug can have more than one code: acetylsalicylic acid (aspirin), for example, has as a drug for local oral treatment,  as a platelet inhibitor, and  as an analgesic and antipyretic. On the other hand, several different brands share the same code if they have the same active substance and indications.

Classification
In this system, drugs are classified into groups at 5 different levels:

First level
The first level of the code indicates the anatomical main group and consists of one letter. There are 14 main groups:

Second level
The second level of the code indicates the therapeutic main group and consists of two digits.

Example: C03 Diuretics

Third level
The third level of the code indicates the therapeutic/pharmacological subgroup and consists of one letter.

Example: C03C High-ceiling diuretics

Fourth level
The fourth level of the code indicates the chemical/therapeutic/pharmacological subgroup and consists of one letter.

Example: C03CA Sulfonamides

Fifth level
The fifth level of the code indicates the chemical substance and consists of two digits.

Example: C03CA01 Furosemide

ATCvet
The Anatomical Therapeutic Chemical Classification System for veterinary medicinal products (ATCvet) is used to classify veterinary drugs. ATCvet codes can be created by placing the letter Q in front of the ATC code of most human medications. For example, furosemide for veterinary use has the code QC03CA01.

Some codes are used exclusively for veterinary drugs, like QI Immunologicals, QJ51 Antibacterials for intramammary use or QN05AX90 amperozide.

Defined daily dose
The ATC system also includes defined daily doses (DDDs) for many drugs. This is a measurement of drug consumption based on the usual daily dose for a given drug. According to the definition, "[t]he DDD is the assumed average maintenance dose per day for a drug used for its main indication in adults."

National adaptions
National issues of the ATC classification, such as the German Anatomisch-therapeutisch-chemische Klassifikation mit Tagesdosen, may include additional codes and DDDs not present in the WHO version.