Anything that irritates the inside of the nose can cause a runny nose. Infections — such as colds, flu or sinusitis — and allergies often cause runny and stuffy noses. Some people have noses that run all the time without a known reason. This is called nonallergic rhinitis or vasomotor rhinitis.

A polyp, an object such as a small toy stuck in the nose, or a tumor might cause the nose to run from only one side. Sometimes migraine-like headaches can cause a runny nose.

Causes of a runny nose include:

1. Acute sinusitis
2. Allergies
3. Chronic sinusitis
4. Churg-Strauss syndrome
5. Common cold
6. Decongestant nasal spray overuse
7. Deviated septum
8. Dry or cold air
9. Granulomatosis with polyangiitis (a condition that causes inflammation of the blood vessels)
10. Hormonal changes
11. Influenza (flu)
12. Object in the nose
13. Medicines, such as those used to treat high blood pressure, erectile dysfunction, depression, seizures and other conditions
14. Nasal polyps
15. Nonallergic rhinitis
16. Pregnancy
17. Respiratory syncytial virus (RSV)
18. Tobacco smoke