Therapeutic action of a few important classes of drugs is given below :

`=>` Over production of acid in the stomach causes irritation and pain.

`=>` In severe cases, ulcers are developed in the stomach.

`=>` Until 1970, only treatment for acidity was administration of antacids, such as sodium hydrogen carbonate or a mixture of aluminium and magnesium hydroxide.

`=>` However, excessive hydrogen carbonate can make the stomach alkaline and trigger the production of even more acid.

`=>` Metal hydroxides are better alternatives because of being insoluble, these do not increase the `color{red}(pH)` above neutrality.

`=>` These treatments control only symptoms, and not the cause.

`=>` Therefore, with these metal salts, the patients cannot be treated easily.

`=>` In advanced stages, ulcers become life threatening and its only treatment is removal of the affected part of the stomach.

`=>` In hyper acidity, a chemical histamine stimulates the secretion of pepsin and hydrochloric acid in the stomach.

● The drug cimetidine (Tegamet), was designed to prevent the interaction of histamine with the receptors present in the stomach wall.

● This resulted in release of lesser amount of acid.

● The importance of the drug was so much that it remained the largest selling drug in the world until another drug, ranitidine (Zantac), was discovered.

`=>` Histamine is a potent vasodilator. It has various functions.

● It contracts the smooth muscles in the bronchi and gut and relaxes other muscles, such as those in the walls of fine blood vessels.

● Histamine is also responsible for the nasal congestion associated with common cold and allergic response to pollen.

`=>` Synthetic drugs, brompheniramine (Dimetapp) and terfenadine (Seldane), act as antihistamines.

● They interfere with the natural action of histamine by competing with histamine for binding sites of receptor where histamine exerts its effect.

`color{red}("Note ")` : Antihistamines do not affect the secretion of acid in stomach because antiallergic and antacid drugs work on different receptors.