A blocked nose is usually caused by swelling and irritation inside the nasal passages (inflammation) and/or extra mucus, most often from infections or allergies. Less commonly, it comes from structural problems in the nose, medication side‑effects, or overuse of decongestant sprays.

Main medical causes

  • Viral infections such as the common cold, flu, COVID‑19 and acute sinusitis irritate the nasal lining, causing it to swell and produce more mucus. This is the most frequent reason for a short‑term blocked nose.
  • Allergic rhinitis (hay fever or dust/pet/mould allergy) makes the immune system release histamine, which leads to sneezing, runny nose and congestion. Symptoms often flare with seasons, dust exposure or contact with pets.
  • Chronic sinusitis and persistent rhinitis cause long‑lasting inflammation in the sinuses and nose, leading to almost constant stuffiness and facial pressure.

Structural and physical factors

  • Deviated septum (a crooked wall inside the nose) narrows one or both nasal passages and can make one side always feel more blocked. This is often due to how the nose grew or to an old injury.
  • Nasal polyps, enlarged adenoids or enlarged turbinates are soft tissue growths or swellings that physically obstruct airflow and cause chronic congestion and mouth breathing. In rare cases, tumours inside the nose or sinuses can present with persistent blockage, especially on one side.

Environment, hormones and medicines

  • Irritants like cigarette smoke, pollution, strong smells, wood dust or chemical fumes can trigger non‑allergic rhinitis with congestion and runny nose. Cold or very dry air can also stuff the nose temporarily.
  • Hormonal changes during pregnancy, puberty or menopause can cause nasal blood vessels to swell, leading to a “pregnancy rhinitis”–type blocked nose in otherwise healthy people. Alcohol and spicy foods may briefly dilate nasal vessels and worsen stuffiness in some.
  • Certain medicines (for example, some blood‑pressure drugs, erectile‑dysfunction medicines, antidepressants and others) list nasal congestion as a side effect. Long‑term overuse of nasal decongestant sprays can cause rebound congestion (rhinitis medicamentosa), where the nose becomes more blocked once the spray wears off.

When it might be serious

  • See a doctor promptly if a blocked nose lasts more than a few weeks without a clear cause, affects sleep or breathing, or is only on one side. Red‑flag signs include nosebleeds, facial pain or swelling, changes to vision, or recurrent sinus infections.
  • Persistent congestion can be linked with snoring and sleep apnoea, which reduce sleep quality and daytime alertness and may need specialist assessment.

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