Causes of runny nose, also called rhinorrhoea may include:
Environment such as cold weather, air conditioning, dry air etc
Sinusitis, acute and chronic
Foods, such as spicy food
Often the symptom of running nose is accompanied by other associated symptoms such as nasal itchiness, stuffiness, obstruction or even bleeding.
A detailed history and examination, including nasal endoscopy may be necessary to make a diagnosis.
Other investigations such imaging studies and allergy tests may be required.