Define drugs and explain their classification.

(N/A) Drugs are chemicals of low molecular masses $(\sim 100-500 \ u)$.
These interact with macromolecular targets and produce a biological response. When the biological response is therapeutic and useful,these chemicals are called medicines and are used in the diagnosis,prevention,and treatment of diseases.
Most of the drugs used as medicines are potential poisons if taken in doses higher than those recommended. The use of chemicals for therapeutic effect is called chemotherapy.
Drugs can be classified mainly on the criteria outlined as follows:
$(a)$ On the basis of 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 a pain-killing effect,and antiseptics kill or arrest the growth of microorganisms.
$(b)$ 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. There are various ways in which the action of histamines can be blocked.
$(c)$ 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,sulfonamides have common structural features as shown below:
$H_2N-C_6H_4-SO_2NHR$
$(d)$ 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. Drugs possessing some common structural features may have the same mechanism of action on targets. The classification based on molecular targets is the most useful classification for medicinal chemists.