What causes cancer?

Very few cancers have a single known cause. Most cancers seem to be caused by a complex mix of many risk factors, but sometimes cancer develops in people who don’t have any risk factors. A risk factor is any substance or condition that increases the risk of developing cancer. Cancer risk factors may play different roles in starting cancer and helping it grow. Examples of risk factors for cancer include:

  • getting older
  • smoking
  • not protecting yourself from the sun
  • having certain genetic changes
  • being overweight or obese
  • not having a healthy diet
  • not getting enough physical activity
  • drinking alcohol
  • coming into contact with harmful chemicals at home or at work
  • having certain types of infections

Your cancer risk

Cancer risk refers to a person’s chance of developing cancer. Having information about cancer risk can help you make informed choices about your health.

In general, the more often and the longer you are in contact with a risk factor, the greater the chance that cancer will develop.

It can take many years for cancer to develop after being around a risk factor. Cancer usually develops after being around many risk factors over time. People may be in contact with several risk factors in the course of their daily lives.

Some people have a higher risk of developing cancer because of certain risk factors. But even if a person has one or more risk factors, it is impossible to know exactly how much these factors may contribute to developing cancer later in life. Being high risk doesn’t mean that cancer will develop.

Even people who have a low risk may still develop cancer. Low risk does not mean that a person will not get cancer. It means that there is less chance of getting cancer. It isn’t always clear why one person gets cancer and another doesn’t.

Studying cancer and what causes it

Over the years, researchers have developed a better understanding of how cancers develop and grow.

They do this by studying people who are in contact with or have been in contact with certain substances or exposures (such as pesticides or exposure to the sun), or people who have certain conditions (such as Lynch syndrome), to see how many develop cancer. This is called cancer epidemiology.

Researchers also study how substances and exposures affect cells and other living organisms. When there is enough scientific evidence to prove that a certain substance or exposure causes cancer, experts will call it a carcinogen. In many cases there isn’t enough information to know for sure whether or not something causes cancer. These substances or exposures may be called probable or possible carcinogens, depending on how much scientific evidence is available or the type of evidence that exists. And sometimes experts can’t say whether a substance is linked to cancer.

There is a lot of misleading and unproven information on the Internet and in the news about what causes cancer or doesn’t cause it. It’s important that you get information from credible organizations that have experts who review the scientific evidence. These include the International Agency for Cancer Research, the US National Toxicology Program and the World Cancer Research Fund.

Experts at the Canadian Cancer Society also make recommendations about cancer-related issues. We can help you find credible information about substances or exposures linked to cancer and help you understand the research.

Cancer risk assessment

People are often concerned about their personal risk of developing cancer. Risk assessment looks at information about a person (such as their age, health history, family history, lifestyle and diet choices) to estimate their cancer risk. Knowing your risk factors can help you make personal health choices to reduce your risk of developing certain cancers. Doctors use a variety of risk assessment tools to estimate a person’s risk of developing cancer.

Your doctor may recommend genetic testing and counselling if they think you may have a higher risk of developing cancer because of a known family cancer syndrome or a family history of cancer.

Understanding your risk of cancer can also help you decide if you may benefit from:

  • starting cancer screening at an earlier age or being screened more often than the general public
  • having surgery or taking medicine to help lower your cancer risk

How to reduce your cancer risk

About 4 in 10 cancer cases can be prevented through healthy living and policies that protect the public. Risk can be increased or decreased by lifestyle choices and the kind of environment a person lives in and works in.

Learn about what you can do to reduce cancer risk.

Expert review and references

Medical disclaimer

The information that the Canadian Cancer Society provides does not replace your relationship with your doctor. The information is for your general use, so be sure to talk to a qualified healthcare professional before making medical decisions or if you have questions about your health.

We do our best to make sure that the information we provide is accurate and reliable but cannot guarantee that it is error-free or complete.

The Canadian Cancer Society is not responsible for the quality of the information or services provided by other organizations and mentioned on cancer.ca, nor do we endorse any service, product, treatment or therapy.

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