There are many possible causes of bleeding in the colon, which is often first detected as blood in the faeces, and usually appears in the toilet bowl or on tissue paper after going to the toilet.*
This sort of bleeding is generally referred to as colonic bleeding or rectal bleeding and depending on the source of the bleed may also be referred to as Lower Gastrointestinal Bleeding (LGIB) or Upper Gastrointestinal Bleeding (UGIB).
Colonic bleeding can be caused by the following conditions:
Cancer of the colon
Of these, the most common cause of colonic bleeding is haemorrhoids, particularly where the blood is bright red, as this indicates that the blood has not travelled very far from its source.
In the case of LGIB, the blood is generally very red in colour, and the higher up the digestive tract the source of the blood, the darker the blood appears. Where this darker blood mixes with the faeces it gives stools a black tarry appearance - these are called 'melena'. Although not always the case, darker blood is four times as likely to be caused by an issue in the upper gastrointestinal tract.
To diagnose the cause of colonic bleeding, the doctor must take into account any previous symptoms, such as change in toilet habits, and colour and consistency of faeces and any tenderness or pain in the abdominal area or in the bowel. At this stage innocent causes of redness, such as the intake of foods that discolour faeces, such as beetroot, dragonfruit / pitaya, iron supplements or bismuth, must be discounted, the next stage being testing the stool for the presence of blood. Finally, a colonoscopy and/or gastroscopy will allow the doctor to internally examine the patient to determine the site of the bleed.
The doctor will also take into account the severity of the bleeding - if the bleeding is mild, i.e. some spots or drops of blood, then this may go away without intervention, although it would still be worth further medical investigation. If however the bleeding is moderate or severe - more than a few teaspoons, then blood loss can cause weakness and dizziness and at this point it is very important to get to hospital quickly.
*another type of colonic bleeding is 'occult' or hidden bleeding i.e. where there is no visible bleeding but blood in very small quantities is in fact present in the faeces. This may be caused by slow blood loss from a higher section of the colon, sometimes caused by polyps or possible by colon cancer. Faeces is tested for this using Faecal Occult Blood Testing (FOBT).