Risk factors

A risk factor is anything that makes it more likely that you will get a disease.

The strongest risk factors for breast cancer are things that you cannot change—for example, your gender, your age, and your genes. Some risk factors, such as being a women, are common; others, such as genetic mutations, are rarer.

There are some risk factors that are under your control. These include drinking alcohol, being physically inactive, and taking hormone replacement therapy. It is less clear whether diet or smoking affect your risk for breast cancer.

Risk factors do not tell you with any certainty whether you will get a disease or not. Most people who have risk factors, even multiple, do not get breast cancer.1 Rather, a risk factor tells you about how much greater your risk is compared to if you didn’t have the risk factor. That is, risk factors are about relative risk.

Risk factors you cannot change

Being a woman

Almost all breast cancer occurs in women.2 Breast cancer in men does happen, but it is rare. It’s estimated that 1 in 8 women will develop breast cancer at some point in their life. An estimated 26,900 Canadian women were diagnosed with breast cancer in 2019.3


Your risk of having breast cancer increases as you age. Most cases of breast cancer (83%) are in women over 50.4 Although breast cancer is rare in young women, it can be devastating if it happens. We have more information about breast cancer in young women.

Family history of breast cancer

If you have one first-degree relative (a parent or a sibling) who has had breast cancer, your risk is about double what it would be otherwise.5 The more first-degree relatives with breast cancer, the greater the risk. However, most people diagnosed with breast cancer have no family history of the disease, and most people who have close relatives with breast cancer never develop it themselves.6

Inherited genetic mutations

About 5-10% of breast cancers are thought to be connected to genetic mutations inherited from parents. When this happens, it’s most often because of a mutation in the BRCA1 or BRCA2 genes.

Normally, BRCA1 and BRCA2 are tumour suppressor genes: their job is to fix errors in DNA replication and prevent cancer from developing. If they have mutations that stop them from doing their job, this can lead to cancer growth.7 Women with these genetic mutations are more likely to have breast cancer when they are younger, and to have it in both breasts.8

There are several other genetic mutations associated with greater risk of breast cancer, including ATM, TP53, CHEK2, PTEN, CDH1, STK11, and PALB2. Recently, a study at the Women’s College Hospital in Toronto has identified a strong association between breast cancer and mutations in a gene called RECQL.9

Having had breast cancer before

If you have had breast cancer before, you’re more likely to develop a new cancer in the opposite breast or in the same breast. This is different from having a recurrence of the original cancer: this is the risk of an entirely new tumour developing.

Radiation exposure

Ionizing radiation increases the risk of breast cancer. If you’ve received radiation therapy to the chest or face before the age of 30 to treat other types of cancer, or to treat Hodgkin’s disease or non-Hodgkin’s lymphoma, your risk of having breast cancer increases.

Note that this is just about radiation used for treatment. When radiation is used for screening or diagnosis, such as screening mammograms, doctors use the lowest level of radiation possible to see what they need to. These levels of radiation do not lead to an increased risk of cancer.

Being overweight after menopause, or gaining weight

Having higher levels of body fat after menopause is linked to greater risk for breast cancer. Gaining weight as an adult is also associated with an increased risk. However, higher levels of body fat before menopause seem to help protect against breast cancer. This does not mean that either losing weight or gaining weight will reduce your risk. Visit Sunnybrook Health Sciences Centre for more information on healthy weight and the link between body fat and breast cancer.

Pregnancy history

Having children reduces your risk of breast cancer. However, being pregnant later in life is associated with a slightly greater risk compared to being pregnant earlier. Women who haven’t had a full-term pregnancy, and women who are pregnant after the age of 30, are more likely to have breast cancer compared to women who gave birth before the age of 30. Breastfeeding can also lower the risk of breast cancer, especially if it continues for more than a year.

Menstrual history

Women who started menstruating before the age of 12, or who have menopause after the age of 55, have a higher risk of breast cancer. This is because the longer you menstruate, the more your body is exposed to estrogen, which in turn increases the risk of breast cancer.

Having dense breasts

Research has shown that having dense breasts makes you more likely to have breast cancer. Dense breasts also make it much harder for mammograms to detect cancer. Find out more about the importance of breast density.

Certain non-cancerous breast conditions

Breast conditions that are benign (not cancer) are divided into three types:

Non-proliferative lesions. These are breast conditions in which cells are not growing excessively (proliferative means growing excessively). They do not seem to affect breast cancer risk substantially. Examples of these conditions include:

  • Fibrocystic breast changes
  • Mild hyperplasia
  • Adenosis (non-sclerosing)
  • Phyllodes tumor (benign)

Proliferative without atypia. In these conditions, cells are growing in excess, but they don’t appear abnormal (atypia means cell abnormalities). These conditions may increase your risk of breast cancer slightly – especially if you also have a family history of breast cancer. They include:

  • Hyperplasia
  • Complex fibroadenoma
  • Sclerosing adenosis
  • Papillomatosis
  • Radial scar

Proliferative with atypia. These breast conditions involve cells growing in excess and some of them look abnormal. Although these are not considered cancer, they are more strongly associated with a greater risk of breast cancer. This is especially so if you also have a family history of breast cancer. They include:

For more information on benign breast conditions and breast cancer risk, see Breast Cancer Risk Factors You Cannot Change (American Cancer Society) and Breast Cancer Risk Factors (Cancer Research UK).

Risk factors you can change

Drinking alcohol

Research consistently shows that drinking alcohol increases your risk of breast cancer. Drinking even one alcoholic drink a day may increase your risk. The risk goes up the more alcohol you consume.10

Using hormone replacement therapy (HRT)

Studies have shown a connection between hormone replacement therapy and increased risk of breast cancer. Women who used hormone replacement therapy with both estrogen and progesterone (combined HRT) for at least five years showed the greatest increase in risk. We know this mostly because of the landmark Women’s Health Initiative study. Since that study’s results were released in 2002, the use of hormone replacement therapy has declined dramatically. Doctors now believe that in most cases, the risks of HRT outweigh the benefits for post-menopausal women.11

Physical inactivity

Being physically inactive is connected with slightly greater breast cancer risk.


Smoking is probably linked to a higher risk of breast cancer in premenopausal women. Heavy secondhand smoke also seems to increase the risk. The evidence is less decisive than for other risk factors, though. Watch this video for more information: Smoking and Breast Cancer – What’s the Link?

Who is most at risk?

If you are at high risk for breast cancer, that does not mean that you are going to get breast cancer. It only means that your likelihood of getting breast cancer in your lifetime is higher.

Each province has different criteria for what they consider high risk. If you meet these criteria in your province, you may be able to get more frequent screening (annually rather than once every two years).

You are high risk if any of these are true for you:

  • You have a genetic mutation associated with greater risk for breast cancer
  • You have a parent, sibling, or child who has a genetic mutation associated with risk for breast cancer
  • Have a family history that indicates a hereditary breast cancer syndrome
  • You have had radiation therapy to your chest before
  • You have been diagnosed with breast cancer before

What are relative risk and absolute risk?

Relative risk is the difference in risk between two groups of people – for example, between people who smoke and people who do not. When a news article says something like, “Breastfeeding lowers the risk for breast cancer”, it is talking about relative risk. Likewise, whenever you hear that something decreases or increases risk of breast cancer, or risk of dying from breast cancer, that is relative risk. Claims like these are based on studies that observe two groups and examines how many of each group get breast cancer.

Here’s the tricky thing about relative risk: If you hear that doing X reduces your risk of breast cancer, and you do X, you might think, “My risk of breast cancer must be low.” Not necessarily: your risk is lower than it would be if you didn’t do X, but that does not tell you if your risk is low overall. Your overall risk is called absolute risk.


  1. National Breast Cancer Coalition. Myths & Truths About Breast Cancer § We don’t know how to prevent breast cancer. Retrieved November 2020 from https://www.stopbreastcancer.org/information-center/myths-truths/
  2. The reason that women are more likely to develop breast cancer is that their breast cells are exposed to the hormones estrogen and progesterone, which are linked with breast cancer – especially estrogen (Canadian Cancer Society, Risk factors for breast cancer).
  3. Public Health Agency of Canada. (2019). Breast Cancer. Retrieved November 2020 from https://www.canada.ca/en/public-health/services/chronic-diseases/cancer/breast-cancer.html
  4. Public Health Agency of Canada. (2019). Breast Cancer.
  5. This is an example of relative risk.
  6. Cancer Research UK. (2019). Family history of breast cancer and inherited genes.
  7. Welcsh, P. L., & King, M. (2001). BRCA1 and BRCA2 and the genetics of breast and ovarian cancer. Human Molecular Genetics 10(7): 705–713. https://doi.org/10.1093/hmg/10.7.705
  8. American Cancer Society. (2019). Breast Cancer Risk Factors You Cannot Change.
  9. Cybulski, C., Carrot-Zhang, J., Klu?niak, W. et al(2015). Germline RECQL mutations are associated with breast cancer susceptibility. Nature Genetics 47643–646. https://doi.org/10.1038/ng.3284
  10. Canadian Cancer Society. Risk factors for breast cancer.
  11. WebMD. (2020). Hormone replacement therapy and breast cancer risk.