• Drugs are chemicals of low molecular masses.
  • They 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.

Drug-Target interaction

  • In the body, proteins act as enzymes in biological reactions and also act as receptors in the body's communication system. In biological reactions, proteins as enzymes facilitate the reaction of the substrate with the reagent. When the substrate comes closer to the enzyme then it binds to the enzyme and then with the help of reagent, chemical reaction happen and then the product is formed.

  • Now, drugs inhibit this activity of the enzyme. Drugs itself bind to the active site of the enzyme and lock the space for the substrate to bind and thus the chemical reaction does not proceed. This inhibition by drugs is done two different ways:

(i) When the drug binds to the enzyme at the active site and thus no space available for the substrate and thus no chemical reaction proceeds.
(ii) When the drug binds to the enzyme at some different site also known as allosteric site, but due to this binding, the shape of the enzyme changes and thus substrate is not able to bind with the enzyme and thus no chemical reaction proceeds.


Drugs can be classified mainly on criteria outlined as follows:

  • On the basis of pharmacological effect It provides the whole range of drugs available for the treatment of a particular type of problem, e.g. analgesics have pain killer 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 the particular biochemical process, e.g. antimicrobials, neurologically active drugs, etc.
  • On the basis of chemical structure Drugs classified in this way share common
    structural features and often have similar pharmacological activity, e.g. sulpha drugs, arsenic drugs.
  •  On the basis of molecular targets Drugs are classified on the basis of their
    interaction with biomolecules such as carbohydrates, lipids, proteins and nucleic acids.
  • Drugs which can block the binding site of the enzyme and prevent the binding of substrate or can inhibit the catalytic activity of the enzyme are called enzyme inhibitors.
  • Drugs which can compete with the natural substrate for their attachment on the active sites of enzymes are called competitive inhibitors.


Pharmacological Classification of Drugs


  • Chemical substances which neutralize excess acid in the gastric juices and give relief from acid indigestion, acidity, heart burns and gastric ulcers. Examples: Eno, gelusil, digene etc.
  • Antacids contain weakly basic chemicals such as magnesium hydroxide i.e. Mg(OH)2, Aluminium hydroxide i.e. Al(Cl)3, magnesium carbonate i.e. MgCO3, etc.
  • One of the most popular antacid tablets is ‘Ranitidine’


  • Chemical substances which diminish or abolish the effects of histamine released in body and hence prevent allergic reactions. .
  • Histamine can cause itching, sneezing, runny nose, and watery eyes.
  • Examples: Brompheniramine (Dimetapp) and terfenadine (Seldane).

Neurologically Active Drugs: Drugs which have a neurological effect i.e. affects the message transfer mechanism from nerve to receptor.


Chemical substances used for the treatment of stress, anxiety, irritability and mild or even severe mental diseases, are known as tranquilizers. These affect the central nervous system and induce sleep for the patients as well as eliminate the symptoms of emotional distress. They are the common constituents of sleeping pills.

Noradrenaline is one of the neurotransmitter that plays a role in mood change. If the level of noradrenaline is low, the signal sending activity becomes low, and the person suffers from depression. In such situations antidepressant drugs are required. These drugs inhibit the enzymes which catalyse the degradation of noradrenaline. If the enzyme is inhibited, this important neurotransmitter is slowly metabolized and can activate its receptor for longer periods of time, thus counteracting the effect of depression. Iproniazid and phenelzine are two such drugs.

Barbituric acid and its derivatives viz. veronal, amytal, nembutal, luminal, seconal are known as barbiturates. Barbiturates are hypnotic, i.e., sleep producing agents.