A drug, also called medicine, is a chemical substance used to cure, prevent, or diagnose a disease. Chemotherapy, use of the chemical substance for the treatment of a disease is termed as chemotherapy.

Classification of Drugs

On the basis of the pharmacological effect

This classification is based on the pharmacological effect of the drugs. It is useful for doctors because it provides them with the whole range of drugs available for the treatment of a particular type of problem.

For example, analgesics have pain killing effect, antiseptics kill or arrest the growth of microorganisms.

• On the basis of drug action
  It is based on the action of a drug on a particular biochemical process. For example, all antihistamines inhibit the action of the compound, histamine which causes inflammation in the body.
• On the basis of chemical structure
  It is based on the chemical structure of the drug. Drugs classified in this way share common structural features and often have similar pharmacological activity. For example, sulphonamides have common structural feature, given below.

• On the basis of molecular targets
  Drugs usually interact with biomolecules such as carbohydrates, lipids, proteins and nucleic acids. These are called target molecules or drug targets.