Nosebleeds can be caused by a number of different things. It's not always possible to identify the exact reason why one occurs.

Nosebleeds can start just inside your nostrils (anterior) or at the back of your nose (posterior). These different types of nosebleeds tend to have different causes.

Anterior nosebleeds

The vast majority of nosebleeds are anterior nosebleeds, which means the bleeding comes from the wall between the two nose channels (the lower septum), just inside your nose. This part of the nose, known as Little's area, contains many delicate blood vessels that can be easily damaged.

The cause of anterior nosebleeds is sometimes unknown, but they can be caused by a number of things, including:

• picking your nose, particularly if you scratch the inside of your nose with a sharp fingernail
• blowing your nose very hard
• a minor injury to your nose
• a blocked or stuffy nose often caused by an infection such as a cold or flu
• sinusitis – an infection of the small, air-filled cavities inside your cheekbones and forehead
• dry air or an increase in temperature drying out the inside of your nose
• hay fever or other allergies
• high altitude
• excessive use of nasal decongestants
• a crooked nose that's either present from birth (congenital) or the result of an injury (a deviated septum)

Anterior nosebleeds are more common in children and are not usually a sign of anything serious. They can often be treated easily at home. Read more about treating nosebleeds.

Posterior nosebleeds

A small number of nosebleeds are posterior nosebleeds, which means the bleeding originates from branches of arteries that supply blood to the space inside your nose between the roof of your mouth and your brain (nasal cavity).

These nosebleeds are more common in adults than children. They can be more serious than anterior nosebleeds and bleed more heavily. Medical attention may be required.

Causes of posterior nosebleeds include:

• a blow to your head, or a fall
• a broken nose
• recent nasal surgery
• hardened arteries (atherosclerosis)
• medicines that cause you to bleed more easily, including aspirin and anticoagulants such as warfarin and heparin
• a tumour in the nasal cavity
• a blood clotting abnormality – such as haemophilia or von Willebrand disease
• hereditary haemorrhagic telangiectasia (HHT) – an inherited genetic condition that affects the blood vessels
• leukaemia (although this is rare and you're likely to have other symptoms as well)

High blood pressure (hypertension) is also more common in people with nosebleeds and may make it harder to stop the bleeding, but it's not clear whether this directly causes nosebleeds.