What Causes Anal Cancer?

Researchers have found some risk factors that increase a person’s risk of anal cancer, but the exact cause of anal cancer is not known.

HPV infection

Most anal cancers seem to be linked to infection with the human papillomavirus (HPV). While HPV infection seems to be important in the development of anal cancer, the vast majority of people with HPV infections do not get anal cancer.

A great deal of research is now being done to learn how HPV might cause anal cancer. There is good evidence that HPV causes many anal squamous cell carcinomas. But the role of this virus in causing anal adenocarcinomas is less certain.

More than 150 subtypes of HPV have been found. The subtype known as HPV-16 is often found in squamous cell carcinoma and is also found in some anal warts. Another subtype, HPV-18, is found less often. Most anal warts are caused by HPV-6 and HPV-11. Warts containing HPV-6 or HPV-11 are much less likely to become cancerous than those containing HPV-16.

HPV makes proteins (E6 and E7) that can shut down 2 important tumor suppressor proteins in normal cells. These proteins – p53 and Rb – normally work to keep cells from growing out of control. When these proteins are not active, cells are more likely to become cancerous.

Lowered immunity

When the body is less able to fight off infections, viruses like HPV can become more active, which might trigger the development of anal cancer. HIV, the virus that causes AIDS, weakens the body’s immune system, as can medicines used to prevent rejection in patients with organ transplants.


Most people know that smoking is the main cause of lung cancer. But few realize that the cancer-causing chemicals in tobacco smoke can travel from the lungs to the rest of the body, causing other types of cancer. Smoking also seems to make the immune system less effective in fighting HPV infections. Many studies have noted an increased rate of anal cancer in smokers, and the effect of smoking is especially important in people with other risk factors for anal cancer.

It’s important to remember that some people with anal cancers do not have any known risk factors and the causes of their cancers are not known.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: November 13, 2017 Last Revised: November 13, 2017

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