A runny nose happens when the lining inside your nose gets irritated or inflamed and starts making extra mucus. That mucus then drains out the front of your nose or down the back of your throat (postnasal drip).
Common everyday causes
The most common triggers are infections like the common cold or flu, which cause your immune system to inflame the nasal passages and ramp up mucus production. Seasonal or year‑round allergies to things like pollen, dust, or pet dander can also inflame the nose and lead to sneezing plus a watery, runny nose.
Other triggers
Nonallergic irritation from cold or dry air, smoke, strong smells, or pollution can make the nose run even when you are not sick or allergic. Some people also get a runny nose from spicy foods or alcohol because these stimulate certain nerves in the nose that increase mucus.
Less obvious medical reasons
Conditions like sinusitis, nasal polyps, or a deviated septum can cause ongoing congestion and a chronic runny nose. Hormone changes, certain medications, and overuse of nasal decongestant sprays can also disturb how the nasal lining works and keep it producing too much mucus.
When to see a doctor
See a doctor urgently if the runny nose is after a head injury, if the fluid is bloody, or if you have high fever, trouble breathing, or feel very unwell. For a runny nose that lasts more than about 10 days, keeps coming back, or really bothers your daily life, a healthcare professional can check for allergies, sinus problems, or other treatable causes.
