Factors Under Study
Many possible risk factors for breast cancer are under study.
Though findings to date are not strong enough to say whether or not these factors are related to breast cancer, research suggests more study is needed.
A full list of factors under study for possible links to breast cancer is too long to present here. The factors below have been studied more than most or they’ve gotten a lot of media attention.
Our Breast Cancer Risk Factors Table compares these and other factors by level of risk and strength of evidence.
The topics below are listed alphabetically. Click on a topic to learn more.
Antibiotic Use
Findings on a potential link between antibiotics and breast cancer risk are mixed.
Some studies have found the use of antibiotics is linked to an increased risk of breast cancer, while others have found no link between the two [385-390].
A pooled analysis of data from 10 studies found some types of antibiotics were linked to an increased risk of breast cancer, while others were not [390]. This may explain some of the differences in findings.
More research is needed to draw conclusions about a possible link between antibiotic use and breast cancer risk.
Antidepressant Use
Most studies, including a meta-analysis that combined the findings from 4 cohort studies, have found no link between antidepressant use and breast cancer [391-394].
However, this topic is still under study.
Aspirin Use
Findings on a potential link between aspirin use and breast cancer are mixed.
Some studies, including 6 meta-analyses, have found a slight decreased risk of breast cancer among women who use aspirin regularly [395-400].
However, other meta-analyses and some cohort studies have found no difference in risk between women who took aspirin regularly compared to those who did not [401-406].
Also, findings from a randomized controlled trial found no difference in breast cancer risk among women who took aspirin every other day for 10 years compared to those who took a placebo [407].
More research is needed to draw conclusions about a possible link between the use of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) and breast cancer risk.
Breast Size
Findings on a potential link between breast size and breast cancer are mixed.
Some studies have found women with larger breasts have a higher risk of breast cancer than women with smaller breasts, while other studies have found no link between breast size and breast cancer risk [408-410].
Two studies found breast size was linked to an increased risk of breast cancer among thin women, but not among heavy women [408-409].
More research is needed to learn whether or not breast size is related to breast cancer risk.
Breastfed as an Infant
Researchers are studying whether having been breastfed as an infant is linked to a lower risk of breast cancer.
Most large studies have found no difference in breast cancer risk between women who were breastfed as an infant and women who were not breastfed [411-415]. However, there are too few studies on this topic to draw conclusions.
Data on other early life exposures are also limited.
Learn more about early life exposures and breast cancer risk.
Learn about breastfeeding and its link to a lower risk of breast cancer in mothers.
Dairy Products
Dairy products are under study as a factor that may be linked to:
- Increased breast cancer risk
- Decreased breast cancer risk
Some researchers have suggested the high fat content in many dairy products or traces of pesticides or growth hormones in milk may be related to an increased risk of breast cancer [416-417].
Others have studied whether the calcium and vitamin D in dairy products may be related to a decreased risk of breast cancer [418].
Learn more about diet and breast cancer.
Dairy products and breast cancer risk after menopause
Large cohort studies and a pooled analysis of data from more than 20 studies have found no link between consuming dairy products (including milk, cheese and yogurt) and the risk of breast cancer after menopause [419-423].
It’s unlikely dairy products are related to breast cancer risk after menopause.
Dairy products and breast cancer risk before menopause
Most studies have found no link between consuming dairy products and the risk of breast cancer before menopause [420-422,424].
However, some findings show women who eat a lot of high-fat dairy products (such as whole milk or butter) may have an increased risk of breast cancer before menopause [420,424].
More research is needed to confirm these findings.
Consuming dairy products during the teenage years or early adulthood
Large cohort studies and a meta-analysis have found no link between eating or drinking dairy products during the teenage years or early adulthood and the risk of breast cancer (before or after menopause) [425-427].
However, this topic is still under study.
Learn more about early life exposures and breast cancer risk.
| For a summary of research studies on dairy products and breast cancer, visit the Breast Cancer Research Studies section. |
DES (in utero exposure) and Breast Cancer Risk among Female Offspring
Diethylstilbestrol (DES) was a drug given to women to prevent pregnancy problems until about 1970. It had estrogen-like traits. DES was taken off the market because it was linked to an increased risk of vaginal cancers in daughters of women who took the drug.
There’s some concern DES might be linked to an increased risk of breast cancer among daughters of women who took the drug during pregnancy.
Studies of women exposed to DES in utero (in the womb) have had mixed findings. Some studies have found an increased risk of breast cancer among women exposed to DES in utero, while others have found no excess risk for these women [415,428-432].
This topic is under study.
Data on other in utero exposures are limited.
Learn more about early life exposures and breast cancer risk.
Diabetes and Insulin
Women with type 2 diabetes may have an increased risk of breast cancer after menopause [83-86]. There doesn’t appear to be an increased risk before menopause [83-84].
The reasons behind a possible link between diabetes and breast cancer after menopause are unclear. It may be related to high levels of insulin in women with type 2 diabetes. Insulin is a hormone that controls blood sugar levels. It’s released by the pancreas after a meal.
Women with other conditions related to high levels of insulin (such as hyperinsulinemia) may also have an increased risk of breast cancer after menopause [433-436].
These issues are under study.
Dietary Fat
Most studies have found no link between a high-fat diet in adulthood and an increased risk of breast cancer [437-443].
However, certain types of fat, rather than the total amount of fat, may be related to breast cancer risk [437-438,440,442].
This topic is under study.
Learn more about diet and breast cancer.
High-fat diet during the teenage years
Although data are limited, eating a high-fat diet during adolescence (the teenage years) may play a role in breast cancer.
Findings from one cohort study showed women who ate a diet high in total fat when they were teens had an increased risk of breast cancer before menopause compared to women who ate a diet low in fat as teens [425].
This topic is under study.
Learn more about early life exposures and breast cancer risk.
| For a summary of research studies on dietary fat and breast cancer, visit the Breast Cancer Research Studies section. |
Fertility Drugs
Fertility drugs stimulate the ovaries, causing estrogen levels in the body to increase. High estrogen levels are linked to an increased breast cancer risk [22]. So, it’s been suggested the use of fertility drugs might be linked to breast cancer risk.
Most studies, including a meta-analysis of 20 studies on the fertility drug clomiphene (Clomid) and 22 studies on other types of fertility drugs, have found no link to breast cancer [444-451].
Some findings with 10 or more years of follow-up suggest fertility drug use may be related to a modest increase in breast cancer risk [448]. However, 2 studies with 20-21 years of follow-up found no increased breast cancer risk among women who used fertility drugs [449,451].
Although most studies show no link between fertility drugs and breast cancer risk, more studies with long-term data are needed to confirm these findings.
Learn more about estrogen and breast cancer risk.
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.
Fiber
Fiber is found in plant foods such as fruits, vegetables, legumes and whole grains. Fiber helps you digest other foods and helps regulate the use of sugar in your body.
Some large studies have found no link between fiber and the risk of breast cancer [452-454]. However, a meta-analysis of 20 studies found women who ate a high-fiber diet had a lower risk of breast cancer than those who did not eat a high-fiber diet [455].
This topic is still under study.
Folate, Folic Acid and Multivitamins
Folate is a vitamin found in many fruits and green leafy vegetables. Folic acid (the supplement form of folate) is added to some breakfast cereals, breads and many multivitamins.
Folate is important in copying and repairing DNA.
Getting enough folate may make it more likely DNA is copied correctly when cells divide. This in turn may make it less likely cells become cancerous. For this reason, folic acid and multivitamins have been studied to see if they might be linked to a lower risk of breast cancer.
Most studies (including 3 meta-analyses) have found no link between blood levels of folate, eating a diet high in folate, taking a folic acid supplement or taking multivitamins and breast cancer risk [456-465].
However, these topics are still under study.
Learn more about diet and breast cancer.
Hair Dyes and Hair Relaxers
Most studies, including a meta-analysis that combined the results of 14 studies, have found no increased risk of breast cancer among women who use permanent hair dye [466-469]. However, some findings suggest women who use permanent hair dye may have a slightly increased risk [468].
One study looked at women’s use of hair dye between 10 and 13 years old and whether it was linked to breast cancer risk when they were adults. It found no increased risk of breast cancer among women who used hair dye between ages 10 and 13 [469].
Hair relaxers
Most but not all studies show no link between the use of hair relaxers and the risk of breast cancer among Black and African American women [468,470-472].
These topics are still under study.
 | For a summary of research studies on hair dyes and breast cancer, visit the Breast Cancer Research Studies section. |
Meat Consumption
It’s been suggested women who eat a lot of meat may have an increased risk of breast cancer. Possible reasons include increased animal fat intake, exposure to chemicals formed when cooking meat at high temperatures and exposure to hormones in meat [417,473].
Most studies, including 2 pooled analyses, have found no link between red meat intake or total meat intake and breast cancer risk [474-476].
However, some findings show women who eat a lot of processed meats, such as sausage and bacon, may have an increased risk of breast cancer [474-477].
These topics are still under study.
Learn more about diet and breast cancer.
Eating meat during the teenage years
Some findings suggest women who ate a lot of red meat during their teenage years may have an increased risk of breast cancer before menopause [478].
However, more study on this topic is needed.
Eating well done or overcooked meat
Studies have looked at whether eating well done or overcooked meat is linked to an increased risk of breast cancer. Some studies have found a link, while others have not [474,479-482].
Eating well done or overcooked meat and breast cancer survival is also under study [483].
| For a summary of research studies on meat consumption and breast cancer, visit the Breast Cancer Research Studies section. |
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.
Moles (on the skin)
Some findings suggest women with a lot of moles on their bodies may have a higher risk of breast cancer than women with few moles [484-485].
The reasons behind a possible link are not clear. However, a high number of moles may be a marker of increased levels of estrogen and testosterone in the body [486]. Increased levels of these hormones are linked to an increased risk of breast cancer [22].
More research is needed before conclusions can be made about a possible link between moles and breast cancer risk.
Learn more about estrogen and breast cancer risk.
Learn more about testosterone (and other androgens) and breast cancer risk.
Parabens (found in some body care products and cosmetics)
Parabens are chemical preservatives found in some cosmetics and body care products. They may be found in some make-up, lotion and shampoo, as well as in foods and medicines.
Parabens have been shown to have very weak estrogen activity. Animal studies have found some negative health effects linked to parabens, but only in very large doses [487-488].
The Cosmetics Ingredient Review Expert Panel conducts research on product safety for the U.S. Food and Drug Administration (FDA).
In 2008, the Panel first concluded paraben exposure from cosmetics and body care products was safe [488]. In 2012, 2017, 2018 and again in 2019, the Panel looked at newer data and concluded paraben exposure from these products was safe [489-493]. Further, a recent study found no increased risk of breast cancer in women with higher paraben exposure (as measured in urine) compared to women with lower paraben exposure [494].
The Panel has also found the estrogen effects from parabens are so weak it’s highly unlikely they can be harmful to health, especially in the small amounts used in cosmetics and body care products [488-493]. The Panel also noted parabens don’t build up in the body over time [488-493].
However, this topic is still under study.
Find more information from the FDA on the safety of products containing parabens.
Komen Perspectives
Read our perspective on body care products containing parabens and breast cancer risk.*
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.
Plastics and Bisphenol A (BPA)
Plastics
Links between plastics and cancer are often reported by the media and in social media (one e-mail hoax falsely claims to be a study from Johns Hopkins University).
However, there’s no scientific evidence supporting a link between using plastic items (such as drinking water from a plastic bottle) and the risk of breast cancer.
Bisphenol A (BPA)
BPA is a chemical found in some plastic containers for food and beverages. The U.S. Food and Drug Administration (FDA) is studying possible health effects from BPA.
Small amounts of BPA from the containers can get into the food and beverages inside. As a result, we can be exposed to low levels of BPA [495]. However, BPA doesn’t build up in the body over time [495].
There’s no evidence to suggest a link between BPA and breast cancer risk [494,496-497].
The FDA continues to study BPA and possible effects on health.
Komen Perspectives
Read our perspective on plastics containing bisphenol A (BPA) and breast cancer risk.*
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.
Secondhand Smoke Exposure (passive smoking)
Most large cohort studies have found no link between breathing secondhand smoke (the smoke from other people’s cigarettes), also called passive smoking, and breast cancer risk [312,498-501].
However, findings from some case-control studies and one large cohort study have shown a slightly increased risk of breast cancer among women with long-term or high levels of secondhand smoke exposure [317,498,502].
More research is needed to draw solid conclusions about a potential link between secondhand smoke and breast cancer risk.
| For a summary of research studies on secondhand smoke exposure and breast cancer, visit the Breast Cancer Research Studies section. |
Soy and Phytoestrogens
Soybeans, as well as other plants like flaxseed, certain grains, beans, fruits and vegetables, contain chemicals called phytoestrogens that mimic estrogen in the body.
Some cell studies have shown soy can increase cancer cell growth [503]. However, many Asian countries (where people eat a lot of soy) have low rates of breast cancer.
This seeming conflict has led to questions about the role of soy in breast cancer risk.
Some studies have found eating a diet high in soy and other phytoestrogens may be linked to a lower risk of breast cancer, while others have found no link [504-512].
Studies in Asian and Asian American women tend to show eating a lot of soy is linked to a decreased risk of breast cancer, but studies in other women tend to show no link between the two [505,507].
These findings may differ because Asian women tend to eat more soy throughout their lives (starting early in life) compared to other women [505,507].
Although eating soy doesn’t appear to be linked to an increased risk of breast cancer, whether it’s linked to a decreased risk remains unclear. This topic is under study.
Learn more about estrogen and breast cancer risk.
Learn more about soy and health.
Learn more about diet and breast cancer.
| For a summary of research studies on soy and breast cancer, visit the Breast Cancer Research Studies section. |
| For a summary of research studies on soy and relief of menopausal symptoms, visit the Breast Cancer Research Studies section. |
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.
Stress
Although one large study found a weak link between life events and breast cancer risk, most studies don’t support a link between stress and breast cancer [513-519].
However, this topic is still under study.
Vitamin D
A possible link between vitamin D and a decreased risk of breast cancer is under study.
Most of the vitamin D a person gets comes from the sun. A small amount comes from diet.
Vitamin D through sunlight exposure and diet
A few studies have found vitamin D exposure (through diet alone or diet plus sunlight) is linked to a lower risk of breast cancer [419,520-524].
However, most studies, including 2 randomized controlled trials and a meta-analysis of 11 studies, have found no link between vitamin D (through diet or supplements) and breast cancer risk [521,523,525-526].
Studying vitamin D presents some challenges. It’s hard to measure sunlight exposure. Also, because so many foods that contain vitamin D also contain calcium, it’s hard to single out the effects of vitamin D alone.
By studying blood levels of vitamin D, researchers can avoid these challenges.
Learn more about diet and breast cancer.
Blood levels of vitamin D
A few studies have found women with higher blood levels of vitamin D had a lower risk of breast cancer compared to women with lower levels [520,527].
However, most studies, including a pooled analysis of more than 10,000 women, have found no link between blood levels of vitamin D and breast cancer risk [528-531].
More research is needed to learn whether there’s a link between vitamin D and breast cancer risk.
Learn more about vitamin D.
| | For a summary of research studies on vitamin D and breast cancer, visit the Breast Cancer Research Studies section. |
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.
Where do the data come from?
Human studies
The data in this section come from 2 main types of research studies:
The goal of these studies is to give information that helps support or disprove an idea about a possible link between an exposure (such as alcohol use) and an outcome (such as breast cancer) in people.
Although they have the same goal, observational studies and randomized controlled trials differ in:
- The way they’re conducted
- The strength of the conclusions they reach
Learn more about different types of research studies.
Animal studies
Animal studies add to our understanding of how and why some factors may be linked to cancer in people.
However, there are many differences between animals and people, so it’s hard to translate findings directly from one to the other.
Animal studies are also designed differently. They often look at exposures in larger doses and for shorter periods of time than are suitable for people.
While animal studies can lay the groundwork for research in people, we need human studies to draw conclusions for people.
All the data you see in this section of the website come from human studies.
Finding information on risk factors
Susan G. Komen® has up-to-date information on many established, probable and possible risk factors for breast cancer. Komen also has information on many factors not related to risk.
Other health organizations conduct research and/or prepare detailed summary reports of research on certain factors shown to have a link (or no link) to breast cancer and other types of cancer, including:
- International Agency for Research on Cancer (IARC)
- Centers for Disease Control and Prevention (CDC)
- National Toxicology Program (NTP)
- U.S. Food and Drug Administration (FDA)
- World Cancer Research Fund International
IARC is a part of the World Health Organization. The CDC, NTP and FDA are all part of the U.S. Department of Health and Human Services. World Cancer Research Fund International is a non-profit organization.
To learn more about the role of the environment in breast cancer, Susan G. Komen® sponsored a study from the National Academies of Sciences, Engineering, and Medicine (formerly the Institute of Medicine), “Breast Cancer and the Environment, a Life Course Approach.” |
Updated 10/4/24