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Possible breast cancer risks

Men and women discussing breast cancer

This page tells you about factors that researchers have looked at, or are looking at, to see if they could possibly increase the risk of getting breast cancer. There is information below about


A quick guide to what's on this page

Possible breast cancer risks

Researchers are investigating some factors that might increase breast cancer risk, including

  • Uneven breasts – One study showed that women who have one breast larger than the other may be at a slightly increased risk of developing breast cancer. But any increase in risk is small compared to other risk factors. For most women it is normal to have slightly uneven (asymmetrical) breasts.
  • Injury to the breast – Scientific research has not identified injury as a risk factor but one study suggested a link. We need more studies to prove or disprove this.
  • Use of antibiotics – A review of studies in 2010 seemed to show that antibiotic use may slightly increase breast cancer risk but we need more studies to clarify this.

Pesticides, having a pregnancy terminated (abortion), and stress have all been investigated as possible causes of breast cancer. But research has found that they don’t seem to increase risk.


CR PDF Icon You can view and print the quick guides for all the pages in the About breast cancer section.



Having uneven breasts

A study in 2006 showed that women who have one breast larger than the other may be at a slightly increased risk of developing breast cancer. No further studies have been carried out to confirm or disprove this. But any increase in risk is small compared to other risk factors. 

For most women it is normal to have slightly uneven (asymmetrical) breasts. More research needs to be done before we know for sure how this may affect breast cancer risk. The important thing is to check your breasts regularly and be aware of how they usually look and feel. And to let your doctor know if you notice any changes, including unevenness.


Injury to the breast

Some women wonder if a past injury could have caused their breast cancer. Generally, doctors don't believe that an injury to the breast can lead to cancer. An injury can cause fibrous scar tissue to form and this may be mistaken for cancer. Or inflammation may make a breast lump that is already there more obvious. There is currently no known link between breast cancer and injury caused by sports, including contact sports.

In 2002, a study took place within the North Lancashire Breast Screening Service. The researchers used questionnaires and interviews to collect information from 67 women with breast cancer and 134 women (the control group) who did not have breast cancer. The study said that the women with breast cancer were more likely to report an injury to the breast in the last 5 years. From their results, the researchers thought there might be a link between injury and cancer. But this is only one study looking at a small group of women in one area. Larger studies are needed to prove or disprove these results.


Bras and breast cancer risk

Although some people have claimed that bras could cause cancer there is no scientific evidence to prove this. Some people have put forward the theory that under wired bras block the fine tubes of the lymphatic system under the arm. The idea is that this stops the normal drainage of the breast tissues, making toxins gather in the breast and resulting in cancer. 

One book suggests that this theory explains the high rate of breast cancer in western cultures and the low rates in the developing world, where women are perhaps less likely to wear bras. But there are no scientific studies that back up this claim. And there are many differences between Western and developing countries' lifestyles that contribute to the differences in breast cancer risk. Much of the research in this area has concentrated on differences in diet between different countries and cultures.

Constriction or applying pressure to an area of the body does not make normal cells become cancerous. We have information about how cancer starts and grows.



Studies looking at whether antibiotic use could increase the risk of breast cancer have shown conflicting results. Antibiotics are drugs used to fight bacterial infections. Some studies seemed to show that antibiotics could increase the risk and others showed no increased risk. 

In 2010 researchers from the University of Athens in Greece pooled results from 5 previous studies and analysed them. They found that use of antibiotics appears to be linked to a slightly higher risk of breast cancer. Although we don’t know for sure, there are several possible reasons why frequent antibiotic use could increase your risk of breast cancer. It is possible that the conditions that led women to take antibiotics in the first place may have caused their increased risk. 

Another possibility is that people who have a weakened immune system, either alone or as well as taking antibiotics, have an increased risk of developing breast cancer. It could also be because antibiotics affect your immune system and how it responds to inflammation, both of which could lead to developing cancer. Some doctors think it could be that antibiotics change the natural bacteria living in your bowel. This may affect the way foods thought to protect against cancer break down in the body.

It is important to remember that antibiotics prescribed by doctors to treat bacterial infections are still very safe and effective. To find out the true reasons for the possible slight increase in breast cancer risk with antibiotic use, we need more research.



Some people think that pesticides increase the risk of breast cancer. Most studies have found that they don't increase cancer risk. But an American report in 2008 found that exposure to the pesticide DDT (dichlorodiphenyltrichloroethane) early in life may increase breast cancer risk. DDT is no longer used but it was widely used from the 1950s to the 1970s.


Having a pregnancy terminated

Pregnancy termination (abortion) has been researched in several different studies. It doesn't seem to increase breast cancer risk. Researchers thought it might, because of the effect of pregnancy on breast cells.



Research has been done into the effects of stress on breast cancer. Many people worry that stress has helped to cause their cancer. But although stress can cause a number of physical health problems, research has not found a definite link between stress and breast cancer. 

A study in Leeds in 1999 looked at stress in women with breast lumps. They wanted to find out if women who had been through a stressful event were more likely to have cancer. Researchers interviewed 332 women who were attending breast clinics in Leeds with a suspicious breast lump. 3 out of every 10 of these women (30%) were diagnosed with breast cancer. The rest had non cancerous lumps. The researchers interviewed all the women and recorded their experiences of stressful life events. There didn't seem to be any link between stress and breast cancer.

A large and very well known study called the Nurses' Health Study published a follow up study on stress in 2004. They were looking particularly at job stress and found no link with breast cancer risk.

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Updated: 12 August 2014