Runny nose (rhinorrhoea): causes, red flags and what helps

The nose makes mucus constantly, trapping dust and germs and warming the air you breathe. When the lining is irritated by a virus, an allergen, smoke, cold air or spicy food, blood flow and secretion rise and the nose runs. Mucus from the sinuses can also drain forwards into the nose, or backwards down the throat as postnasal drip.

Because the process is the same for many triggers, the clues to the cause lie in what comes with the discharge: sneezing and itchy eyes suggest allergy, sore throat and tiredness suggest a cold, facial pain and pressure suggest sinusitis, and unilateral discharge or discharge after injury points to a different category of problem altogether.

A cold and flu both cause a runny nose, but the NHS says flu tends to be more severe, appears within a few hours and leaves you exhausted and unable to carry on as normal, while a cold affects mainly your nose and throat and leaves you able to carry on.

Many people assume that green or yellow mucus means a bacterial infection needing antibiotics. The NHS does list green or yellow mucus among sinusitis symptoms, but it also says antibiotics are rarely needed for sinusitis because it is usually caused by a virus rather than bacteria. Colour mostly reflects white blood cells passing through the mucus during any inflammation, including colds, so it is a poor guide on its own.

Duration, severity and associated features are more helpful: improvement over the first week suggests a viral illness, while worsening after initial improvement, a high temperature lasting beyond a few days or marked facial pain deserves review.

• Call 999 if clear fluid is coming from the nose or ears after a head injury, or there is bruising behind the ears, as the NHS head injury advice lists.

• Call 999 for severe difficulty breathing, swelling of the lips, mouth, throat or tongue, or a tight throat, which can accompany a serious allergic reaction.

• Ask for an urgent GP appointment or NHS 111 help if sinusitis symptoms come with feeling very unwell, painkillers not helping or symptoms getting worse, or a weakened immune system, for example during chemotherapy.

• See a GP if cold symptoms do not get better after 10 days, you have a high temperature for more than 3 days, you feel short of breath or have chest pain, or you have a long-term medical condition or weakened immune system.

• A runny nose from one nostril only, particularly if blood-stained or foul-smelling, or with a blocked nose on the same side, needs examination; the NHS says a GP may refer to ENT if symptoms affect only 1 side of the face.

• See a GP if symptoms are getting worse or do not improve after taking hay fever medicines from the pharmacy.

• Seek advice for babies and young children, who may have difficulty feeding and sleeping with a cold; the NHS says to call 111 for advice about a child under 5 who is unwell.

Questions cover how long the discharge has lasted, which side, whether it is watery, thick or blood-stained, any sneezing or itch, seasonal pattern, asthma or eczema, triggers such as pets, dust or smoke, previous head injury or nasal surgery, and any smell loss, snoring or facial pain. The clinician looks inside the nose with a light, checks the throat, ears and sinuses and listens to the chest if there is a cough.

Allergy testing is considered when hay fever or perennial allergy is suspected and not controlled; ENT referral with nasal endoscopy is considered for persistent, recurrent or one-sided symptoms, and the NHS says a GP can usually spot nasal polyps by looking inside the nose.

For a cold the NHS suggests rest, plenty of fluids, healthy food and steam from a hot shower for a blocked nose, and says a pharmacist can advise on paracetamol or ibuprofen and on decongestant sprays, drops or tablets, which should not be used by children under 6. Nasal sprays should not be used for more than a week because they can worsen symptoms. It notes there is little evidence that vitamin C, echinacea or garlic prevent colds or speed recovery.

For hay fever the NHS suggests petroleum jelly around the nostrils, wraparound sunglasses, showering and changing clothes after being outside, keeping windows shut, damp dusting and avoiding cutting grass. Pharmacists can suggest antihistamine drops, tablets or nasal sprays and steroid nasal sprays, and non-drowsy antihistamines if sleepiness is a problem. If these do not work a GP may prescribe a steroid treatment or refer for immunotherapy.

For sinusitis, a salt water nose rinse, painkillers and avoiding allergy triggers are suggested, with steroid sprays or antihistamines if needed. Stay at home while you have a high temperature or feel unwell, to avoid spreading infection.