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what causes runny nose

A runny nose happens when the lining of your nose gets irritated or inflamed and starts producing extra mucus that then drips out of your nostrils or down your throat (postnasal drip).

Main causes of a runny nose

Think of your nose as a self-cleaning air filter; whenever it senses “trouble,” it turns up the mucus tap.

1. Infections (very common)

  • Common cold and flu : Viruses irritate the nasal lining, causing inflammation and extra mucus, often with sore throat, cough, mild fever, and fatigue.
  • Other respiratory infections: COVID‑19, RSV, sinusitis, bronchitis, and pneumonia can all trigger a runny nose, especially early in the illness.

When you get sick, your immune system sends more blood and fluid to the area to fight germs, which is why your nose floods.

2. Allergies (hay fever and more)

  • Allergens like pollen, dust, pet dander, or mold can make your immune system overreact as if they were dangerous.
  • This reaction releases histamine, causing sneezing, itchy eyes, and a clear , watery runny nose that often flares in certain seasons or environments.

People often notice: “Every spring my nose just won’t stop running” – that pattern strongly suggests allergies.

3. Nonallergic (irritant) rhinitis

Not all runny noses are allergies or infections.

  • Triggers include smoke, strong smells (perfume, cleaning products), pollution, weather changes, cold or dry air, and spicy foods.
  • The lining of the nose is simply irritated, so it produces more mucus even though your immune system isn’t fighting germs or classic allergens.

This is often called nonallergic rhinitis and tends to cause chronic or recurring runny nose without obvious allergy test results.

4. Structural or long‑term nose problems

Sometimes the “hardware” of your nose is part of the problem:

  • Sinusitis (acute or chronic) : Inflammation and infection in the sinuses cause congestion plus thick or discolored mucus that can drip from the nose or down the throat.
  • Deviated septum : When the wall between your nostrils is crooked, airflow is disturbed, which can lead to repeated infections and ongoing runny nose.
  • Nasal polyps : Soft growths inside the nose or sinuses can block drainage and cause persistent congestion and runny nose.

These causes are more likely if symptoms drag on for months, affect just one side more, or don’t improve with usual cold/allergy care.

5. Medications and hormones

Your nose can react to changes inside your body too.

  • Medicines : Some blood pressure drugs, antidepressants, erectile‑dysfunction medicines, and long‑term overuse of decongestant nasal sprays can all lead to a constant runny nose.
  • Hormonal changes : Pregnancy, thyroid changes, or other hormone shifts can cause the nasal blood vessels to swell and leak more fluid, leading to “pregnancy rhinitis” or similar symptoms.

People sometimes think they “just have allergies” when in reality a new medication or pregnancy is the underlying driver.

6. Environment, food, and lifestyle triggers

  • Weather changes : Moving from warm to cold environments, especially into cold dry air, can temporarily make your nose run.
  • Spicy foods : Hot peppers and strong spices can trigger “gustatory rhinitis,” a short‑lived watery runny nose during or after eating.
  • Smoke or pollution : These irritants inflame the nasal lining, causing both stuffiness and dripping.

A classic example is your nose running when you step out into cold winter air or eat a very spicy meal.

7. Less common but important causes

  • Foreign body (object in the nose) : More common in children; often causes a foul‑smelling discharge, sometimes just from one nostril.
  • Underlying systemic diseases : Rare conditions like granulomatosis with polyangiitis or Churg‑Strauss syndrome can involve chronic nasal inflammation and runny nose as part of a larger illness.
  • Cerebrospinal fluid (CSF) leak : Very rare; usually a clear, watery drip from one nostril that worsens leaning forward after head trauma or sinus surgery, and it needs urgent evaluation.

These are not common, but doctors consider them if symptoms are unusual, one‑sided, or associated with other serious signs.

When a runny nose is more serious

You should seek medical help promptly if:

  • Runny nose lasts more than 10–14 days or keeps returning without a clear reason.
  • You have high fever, facial pain or swelling, shortness of breath, or chest pain along with nasal symptoms.
  • Only one side of the nose runs, especially if the fluid is very watery and persistent, or if there is a bad smell or blood.
  • A child might have put something in their nose.

Doctors can usually narrow down what causes runny nose in your case by asking how long it has lasted, what triggers it, what the mucus looks like, and what other symptoms you have.

Information gathered from public forums or data available on the internet and portrayed here.