`=>` Chemistry has influenced our life so much that we do not even realise that we come across chemicals at every moment; that we ourselves are beautiful chemical creations and all our activities are controlled by chemicals.

`=>` The principles of chemistry have been used for the benefit of mankind.

`=>` Think of cleanliness — the materials like soaps, detergents, household bleaches, tooth pastes, etc. will come to your mind.

`=>` If we look towards the beautiful clothes —cchemicals of the synthetic fibres used for making clothes and chemicals giving colours to them will come to your mind.

`=>` Sickness and diseases remind us of medicines — again chemicals.

`=>` Explosives, fuels, rocket propellents, building and electronic materials, etc., are all chemicals.

`color{green}("Drugs ")` : Drugs are chemicals of low molecular masses (`~100 – 500u`).

● These interact with macromolecular targets and produce a biological response.

`color{green}("Medicines ")` : When the biological response is therapeutic and useful, these chemicals are called medicines and are used in diagnosis, prevention and treatment of diseases.

`color{green}("Note ")` : If taken in doses higher than those recommended, most of the drugs used as medicines are potential poisons.

● Use of chemicals for therapeutic effect is called chemotherapy.

Drugs can be classified mainly on criteria outlined as follows :

`=>` This classification is based on pharmacological effect of the drugs.

`=>` It is useful for doctors because it provides them 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.

`=>` 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 action of histamines can be blocked.

`=>` 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 in fig.

`=>` 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.