Table III.

The Anatomical Therapeutic Chemical classification of drugs

Hierarchical levels	Digit(s) and meaning
Level I	A letter of the alphabet indicating the main anatomical group, i.e. the anatomical site of action of the drug*
Level II	Two numbers indicating the main therapeutic group to which the drug belongs
Level III	A letter of the alphabet indicating the pharmacological therapeutic subgroup, according to the different mechanisms/site of action
Level IV	A letter of the alphabet indicating the chemical subgroup in the frame of the pharmacological therapeutic subgroup
Level V	A two-digit number referred to the specific chemical substance

ATC: Anatomical Therapeutical Chemical.

^*Main anatomical groups are 14: A) Gastrointestinal system and metabolism; B) Blood and hematopoietic organs; C) Cardiovascular system; D) Dermatological drugs; G) Genitourinary system and sex hormones; H) Systemic hormonal preparations, excluding sex hormones and insulin; J) General antimicrobials for systemic use; L) Antineoplastic and immunomodulatory drugs; M) Musculoskeletal system; N) Nervous system; P) Insecticidal and repellent antiparasitic drugs; R) Respiratory system; S) Sense organs; V) Various.