There are many possible causes. The commonest is damage to a blood vessel in the nasal mucous membrane caused by an injury, picking the nose, or blowing it too vigorously. It sometimes occurs with no clear cause, particular in patients with blood clotting problems or those who use anticoagulants. A nosebleed should be stopped as follows: blow the nose well once (to remove blood clots that may inhibit sealing of the affected vessel). Sit upright and hold the nose closed for at least ten minutes. If bleeding has not stopped after half an hour, a doctor should be consulted. If necessary the affected vessel can be sealed by cauterization.