Runny nose can be seen in allergic rhinitis, common cold, cerebrospinal fluid nasal leakage and other conditions. 1. Allergic rhinitis: if the runny nose is accompanied by paroxysmal sneezing and itchy nose, it is mostly caused by allergic rhinitis, mainly due to contact with allergic causes, such as special smell, dust, pollen, etc., which may lead to an allergic reaction in the nasal cavity, and the increase of secretion causes the runny nose. 2. Common cold: It is usually caused by viral infections, such as rhinovirus, adenovirus, etc., which stimulate the mucous membrane of the nasal cavity, resulting in increased nasal mucous membrane secretion, accompanied by congestion and swelling of the mucous membrane of the nasal cavity. 3. Cerebrospinal fluid leakage: it can be caused by craniocerebral trauma and medical injury, resulting in intermittent or persistent discharge of clear watery fluid from the nasal cavity, or it can be regarded as runny nose. There may be other causes of runny nose. If the symptoms continue unabated, you should seek prompt medical attention to identify the cause.