The classification categorises substances at five different levels according to the organ or system on which they act and their chemical, pharmacological and therapeutic properties. There are fourteen main groups (1^st level), with one pharmacological/therapeutic subgroup (2^nd level). The 3^rd and 4^th levels are chemical/pharmacological/therapeutic subgroups and the 5^th level represents the chemical substance. The use of anatomical, therapeutic and pharmacological levels is, however, not strictly organised. Table 1 shows the first level ATC categories.

Table 1 Main groups of the Anatomical Therapeutic Chemical classification

ATC codes are alphanumerical and use 7 positions. The character in the 4^th position makes it possible to distinguish an ATC code from an ICD-10 code. ICD-10 never uses alphabetic characters in this position but ATC always does. An example of the code structure is given in Table 2.

Table 2 Example of ATC code structure (metformin)

Medicinal products are primarily classified according to the main therapeutic use of the main active ingredient. The basic principle is one ATC code for each pharmaceutical formulation (i.e. similar ingredients, strength and pharmaceutical form). A substance can be given more than one ATC code if it is available in two or more strengths or formulations with clearly different therapeutic uses. It is important to note that a medicinal product may be used for more than one equally important indication and the main therapeutic use of a drug may differ between countries. In these situations there are often several classification alternatives. Problems such as these are discussed in the WHO International Working Group for Drug Statistics Methodology where the final classification is decided. The principles of classification are further described in the guidelines to ATC classification (1).