Pharmacopœia Pharmacometrika

Drug classification reference using the ATC system with mechanism of action, drug classes, and pharmacological properties.
Published

November 25, 2025

Pharmacopœia: A collection of directions for the identification of medicines.

A drug can be classified two ways: by what it treats (ATC) and by what it is (modality). This section provides a high-level overview of drugs, organized by the ATC classification system.

ATC

The Anatomical Therapeutic Chemical (ATC) classification system was developed in Norway and was introduced in 1976 and is maintained by the World Health Organization Collaborating Centre for Drug Statistics Methodology (WHOCC). It classifies active substances based on the organ or system they act on, as well as their therapeutic, pharmacological, and chemical properties.

The complete ATC classification of metformin illustrates the structure of the code:

Table 1: ATC classification of metformin
Level Code Description
1: Anatomical main group A Alimentary tract and metabolism
2: Therapeutic subgroup A10 Drugs used in diabetes
3: Pharmacological subgroup A10B Blood glucose lowering drugs, excl. insulins
4: Chemical subgroup A10BA Biguanides
5: Chemical substance A10BA02 Metformin

Thus, in the ATC system all plain metformin preparations are given the code A10BA02. Metformin’s modality would be a small molecule. For the chemical substance, the International Nonproprietary Name (INN) is preferred. If INN names are not assigned, USAN (United States Adopted Name) or BAN (British Approved Name) names are usually chosen.

Drugs are classified in groups at five different levels.

  • ATC 1st level: Main anatomical or pharmacological group, shown in Table 3 below.
  • ATC 2nd level: Pharmacological or Therapeutic subgroup
  • ATC 3rd & 4th levels: Chemical, Pharmacological or Therapeutic subgroup
  • ATC 5th level: Chemical substance
Table 2: ATC count per level
ATC level Codes Different names/pharmaceuticals
1: Anatomical main group 14 14
2: Therapeutic subgroup 94 94
3: Pharmacological subgroup 267 262
4: Chemical subgroup 889 819
5: Chemical substance 5067 4363

The 2nd, 3rd and 4th levels are often used to identify pharmacological subgroups when that is considered more appropriate than therapeutic or chemical subgroups.

Table 3: Codes in the first ATC level, the Anatomical main group
ATC code Contents
1 A Alimentary tract and metabolism
2 B Blood and blood forming organs
3 C Cardiovascular system
4 D Dermatologicals
5 G Genito-urinary system and sex hormones
6 H Systemic hormonal preparations, excluding sex hormones and insulins
7 J Antiinfectives for systemic use
8 L Antineoplastic and immunomodulating agents
9 M Musculo-skeletal system
10 N Nervous system
11 P Antiparasitic products, insecticides and repellents
12 R Respiratory system
13 S Sensory organs
14 V Various

Modality

In drug development, a drug modality (Table 4) is the broad “type” or “format” of therapeutic intervention defined primarily by its molecular (or cellular) nature and the way it produces a clinical effect. Modality is orthogonal to ATC, a drug has both an ATC code and a modality.

In practice: modality = “What kind of therapeutic thing is this?”

Table 4: Modality categories
Category Modalities
Conventional Small molecules
Antibodies Monoclonal antibody (mAb), Antibody–Drug Conjugate (ADC), Bi-specific antibody (BsAb)
Proteins and peptides Recombinant
Cell therapies Cell therapies, CAR-T, Stem cells, TCRT, CAR-NK, TILs
Gene therapies Gene augmentation, Gene editing
Nucleic acids DNA and RNA therapies, RNAi, mRNA
Other new modalities Oncolytic viruses, Microbiomes, PROTAC

ATMP (Advanced Therapy Medicinal Product) is another term that is being used to describe a group of modalities. In essence, ATMPs are a class of medicines that use living materials like cells, tissues, or genetic material to treat or prevent diseases.