Explanation:

(1) On the basis of chemical structure: Drugs have been classified on the basis of their chemical structures because drugs having common structural features often have similar pharmacological activity.

e.g., all sulphonamides having the common structural feature as given below are mostly antibacterial.

(2) On the basis of drug action: This classification is based on the action of a drug on a particular biochemical process.

(3) On the basis of molecular target: Drugs usually interact with the biomolecules or biological macromolecules such as proteins, nucleic acids, and lipids. These are called drug targets.

Drugs possessing some common structural features may have the same mechanism of action on a specific drug target. This classification is most useful for medicinal chemists.

(4) On the basis of pharmacological effect: This classification is based upon the pharmacological effects of the drugs.

It is more useful for the doctors because it provides them the whole range of drugs available for the treatment of a particular disease. e.g., analgesics reduce or kill pain while antiseptic either kill or arrest the growth of microorganisms.