A drug class is a group of medications that share certain similarities. Drugs can be classified in several ways, including by:

• Chemical structure
• Mechanism of action
• Therapeutic use
• Organ system affected
• Formulary category
• Abuse potential

ATC classification

The World Health Organization (WHO) drug classification system is called the Anatomical Therapeutic Chemical (ATC) classification.

In the ATC system, active substances are organized into a hierarchy based on:

• The organ or body system they act on
• Their therapeutic, pharmacological, and chemical properties

Drugs are classified at five levels (levels 1 to 5). The system includes fourteen main anatomical/pharmacological groups at the 1st level, and each main group is further divided into 2nd levels.

ATC classification of medications

ATC level	Basis of classification
1st level	The system has fourteen main anatomical or pharmacological groups
2nd level	Pharmacological or therapeutic subgroup
3rd and 4th levels	Chemical, pharmacological or therapeutic subgroup
5th level	Chemical substance

The ATC 1st levels

ATC level	Basis of classification
A	Alimentary tract and metabolism
B	Blood and blood-forming organs
C	Cardiovascular system
D	Dermatologicals
G	Genitourinary system and sex hormones
H	Systemic hormonal preparations, excluding sex hormones and insulin
J	Anti infective for systemic use
L	Antineoplastic and immunomodulating agents
M	Musculoskeletal system
N	Nervous system
P	Antiparasitic products, insecticides, and repellents
R	Respiratory system
S	Sensory organs
V	Various

For example, metformin is used in diabetes mellitus to help control blood glucose levels. Metformin is assigned the code A10BA02, which breaks down like this:

A - for 1st level, Alimentary tract and metabolism

A10 - for 2nd level, therapeutic subgroup, Drugs used in diabetes

A10B - for 3rd level, pharmacological subgroup, Blood glucose lowering drugs, excl. insulins

A10BA - for 4th level, chemical subgroup, Biguanides

A10BA02 - for 5th level, chemical substance, metformin

USP classification

The United States Pharmacopeia (USP) sets standards for the quality, purity, and strength of drugs, supplements, and food ingredients. It is recognized by the FDA.

In pharmacy practice, USP standards are commonly used for:

• Quality control
• Regulatory compliance
• Compounding practices
• Guidance on labeling and storage best practices

The USP and the National Formulary (NF) together publish the USP-NF, which provides quality standards for medical and pharmacological products.

The USP also classifies medications into 47 broad categories, based on therapeutic use, mechanism of action, and formulary classification. This classification supports prescribing under the Medicare Prescription Drug Benefit. Examples of categories include analgesics, antivirals, and cardiovascular agents.

An example of USP classification

Therapeutic category	Pharmacologic class	Formulary key drug type
Antibacterial	Macrolide	Erythromycins

The following USP chapters are commonly referred in the pharmacy-

USP<795> This chapter contains the standards for non- sterile compounding.

USP<797> This chapter contains the standards for sterile compounding.

USP<800> This chapter contains the standards for the safe handling of hazardous drugs.

A formulary (also called a drug list) is a list of prescription drugs covered by a prescription drug plan or another insurance plan that offers prescription drug benefits.