What causes colorectal cancer?

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A cancerous tumour starts from one abnormal cell. In the case of colorectal cancer, it is a cell in a polyp which is the most common site for a cancerous cell to develop. The exact reason why a cell becomes cancerous is unclear. It is thought that something damages or alters certain genes in the cell. This makes the cell abnormal and multiply 'out of control'.

The exact reason why a cell becomes cancerous is unclear. It is thought that something damages or alters certain genes in the cell. This makes the cell abnormal and multiply 'out of control'.

See separate leaflet called 'Cancer - What Causes Cancer' for more details.

Risk factors
Although colorectal cancer can develop for no apparent reason, there are certain 'risk factors' which increase the chance that colorectal cancer will develop. These risk factors include:

Ageing. Colorectal cancer is more common in older people. Eight out of ten people who are diagnosed with colorectal cancer are older than 60 years.
If a close relative has had colorectal cancer (there is some genetic factor).
If you have familial adenomatous polyposis or hereditary non-polyposis colorectal cancer. However, these are rare inherited disorders.
If you have ulcerative colitis or Crohn's disease (conditions of the colon) for more than 8-10 years.
Obesity.
Lifestyle factors: little exercise, drinking a lot of alcohol.

Protective factors
There is a reduced risk of developing colorectal cancer in:

Women who take hormone replacement therapy (HRT).
People who eat a lot of fruit and vegetables.
People who take anti-inflammatory medicines, for example, aspirin. However, there are other risks of taking aspirin (for example bleeding from the stomach) so it is not recommended that people take it to solely lower their risk of colorectal cancer.