You should call an ambulance or go directly to the nearest emergency department if you bring up (vomit) blood. Often the bleeding will stop quite quickly but in some cases it can become severe and life-threatening.
So always play safe and seek medical help quickly. There is a range of different causes - discussed below. Many causes can be treated but the first priority is to make sure the bleeding stops. The rest of this leaflet aims to give some background information but is not a substitute for obtaining immediate medical attention if you vomit blood.