Explain the therapeutic action of antacids and antihistamines.

(N/A) Antacids: Overproduction of acid in the stomach causes irritation and pain. In severe cases,ulcers are developed in the stomach. Until $1970$,the only treatment for acidity was the administration of antacids,such as sodium hydrogen carbonate or a mixture of aluminum 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,being insoluble,they do not increase the $pH$ 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 the only treatment is the removal of the affected part of the stomach.
$A$ major breakthrough in the treatment of hyperacidity came through the discovery that 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 the release of a lesser amount of acid.
The importance of the drug was so great that it remained the largest-selling drug in the world until another drug,ranitidine (Zantac),was discovered.
$(b)$ Antihistamines: 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 the common cold and allergic response to pollen. Synthetic drugs,brompheniramine (Dimetapp) and terfenadine (Seldane),act as antihistamines.