Runny nose occurs when nasal tissues and blood vessels produce excess fluid or mucous that runs out of your nose or down the back of your throat. Runny nose can be caused by a cold, influenza, allergies to such things as dust, pollen or pet dander, or as a response to irritants such as tobacco smoke. Other irritants, such as spicy foods, also can trigger a runny nose. Some people have a chronically runny nose for no apparent reason — a condition called nonallergic rhinitis. Runny nose may or may not be accompanied by nasal congestion.
A runny nose may be annoying and uncomfortable, but it usually clears up on its own. Sometimes a runny nose indicates a more serious problem.