Antihistamines interfere with the natural action of histamines by competing with histamine for binding sites of receptor. Hence, they act as antacid and antiallergic. e.g. Seldane, dimetapp, cimetidine. Histamine stimulates the secretion of pepsin and hydrochloric acid in the stomach. It is also responsible for nasal congestion associated with common cold and allergic response to pollen.