Smoking tobacco appears to increase the risk of breast cancer, with the greater the amount smoked and the earlier in life that smoking began, the higher the risk. In those who are long-term smokers, the risk is increased 35% to 50%. A lack of physical activity has been linked to about 10% of cases. Sitting regularly for prolonged periods is associated with higher mortality from breast cancer. The risk is not negated by regular exercise, though it is lowered.

There is an association between use of hormonal birth control and the development of premenopausal breast cancer, but whether oral contraceptives use may actually cause premenopausal breast cancer is a matter of debate. If there is indeed a link, the absolute effect is small. Additionally, it is not clear if the association exists with newer hormonal birth controls. In those with mutations in the breast cancer susceptibility genes "BRCA1" or "BRCA2", or who have a family history of breast cancer, use of modern oral contraceptives does not appear to affect the risk of breast cancer.

The association between breast feeding and breast cancer has not been clearly determined; some studies have found support for an association while others have not. In the 1980s, the abortion–breast cancer hypothesis posited that induced abortion increased the risk of developing breast cancer. This hypothesis was the subject of extensive scientific inquiry, which concluded that neither miscarriages nor abortions are associated with a heightened risk for breast cancer.

A number of dietary factors have been linked to the risk for breast cancer. Dietary factors which may increase risk include a high fat diet, high alcohol intake, and obesity-related high cholesterol levels. Dietary iodine deficiency may also play a role. Evidence for fiber is unclear. A 2015 review found that studies trying to link fiber intake with breast cancer produced mixed results. In 2016 a tentative association between low fiber intake during adolescence and breast cancer was observed.

Other risk factors include radiation and shift-work. A number of chemicals have also been linked, including polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and organic solvents Although the radiation from mammography is a low dose, it is estimated that yearly screening from 40 to 80 years of age will cause approximately 225 cases of fatal breast cancer per million women screened.

In some population studies moderate alcohol consumption is associated with increase the breast cancer risk.

In contrast, research by the Danish National Institute for Public Health, comprising 13,074 women aged 20 to 91 years, found that moderate drinking had virtually no effect on breast cancer risk.

Studies that control for screening incidence show no association with moderate drinking and breast cancer, e.g.. Moderate drinkers tend to screen more which results in more diagnoses of breast cancer, including mis-diagnoses. A recent study of 23 years of breast cancer screening in the Netherlands concluded that 50% of diagnoses were over-diagnoses.

Compared to other diseases or other cancers, breast cancer receives a proportionately greater share of resources and attention. In 2001 MP Ian Gibson, chairman of the House of Commons of the United Kingdom all party group on cancer stated "The treatment has been skewed by the lobbying, there is no doubt about that. Breast cancer sufferers get better treatment in terms of bed spaces, facilities and doctors and nurses." Breast cancer also receives significantly more media coverage than other, equally prevalent cancers, with a study by Prostate Coalition showing 2.6 breast cancer stories for each one covering cancer of the prostate. Ultimately there is a concern that favoring sufferers of breast cancer with disproportionate funding and research on their behalf may well be costing lives elsewhere. Partly because of its relatively high prevalence and long-term survival rates, research is biased towards breast cancer. Some subjects, such as cancer-related fatigue, have been studied little except in women with breast cancer.

One result of breast cancer's high visibility is that statistical results can sometimes be misinterpreted, such as the claim that one in eight women will be diagnosed with breast cancer during their lives—a claim that depends on the unrealistic assumption that no woman will die of any other disease before the age of 95. This obscures the reality, which is that about ten times as many women will die from heart disease or stroke than from breast cancer.

The emphasis on breast cancer screening may be harming women by subjecting them to unnecessary radiation, biopsies, and surgery. One-third of diagnosed breast cancers might recede on their own. Screening mammography efficiently finds non-life-threatening, asymptomatic breast cancers and pre-cancers, even while overlooking serious cancers. According to H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice, research on screening mammography has taken the "brain-dead approach that says the best test is the one that finds the most cancers" rather than the one that finds dangerous cancers.

Studies suggest that drinking alcohol during pregnancy may affect the likelihood of breast cancer in daughters. "For women who are pregnant, ingestion of alcohol, even in moderation, may lead to elevated circulating oestradiol levels, either through a reduction of melatonin or some other mechanism. This may then affect the developing mammary tissue such that the lifetime risk of breast cancer is raised in their daughters."

It occurs in all adult age groups. While the majority of patients are between 40 and 59 years old, age predilection is much less pronounced than in noninflammatory breast cancer. The overall rate is 1.3 cases per 100000, black women (1.6) have the highest rate, Asian and Pacific Islander women the lowest (0.7) rates.

Most known breast cancer risk predictors do not apply for inflammatory breast cancer. It may be slightly associated with cumulative breast-feeding duration.

Breast cancer risk is elevated for defined fraction of lesions. Except for patients with a strong family history of breast cancer, where the risk is two-fold, nonproliferative lesions have no increased risk. Proliferative lesions also have approximately a 2-fold risk. In particular, atypical hyperplasia is associated with an increased risk of developing breast cancer. Atypical lobular hyperplasia is associated with the greatest risk, approximately 5-fold and especially high relative risk of developing premenopausal breast cancer. Atypical ductal hyperplasia is associated with 2.4-fold risk. In contrast, a New England Journal of Medicine article states that for women with a strong familial history of breast cancer, the risk of future breast cancer is roughly doubled, independent of histological status. The article further states "The relative risk of breast cancer for the cohort was 1.56 (95 percent confidence interval, 1.45 to 1.68), and this increased risk persisted for at least 25 years after biopsy. The relative risk associated with atypia was 4.24 (95 percent confidence interval, 3.26 to 5.41), as compared with a relative risk of 1.88 (95 percent confidence interval, 1.66 to 2.12) for proliferative changes without atypia and of 1.27 (95 percent confidence interval, 1.15 to 1.41) for nonproliferative lesions. The strength of the family history of breast cancer, available for 4808 women, was a risk factor that was independent of histologic findings. No increased risk was found among women with no family history and nonproliferative findings. In the first 10 years after the initial biopsy, an excess of cancers occurred in the same breast, especially in women with atypia."

It is not well understood whether the lesions are precursors of breast cancer or only indication of increased risk, for most types of lesions the chance of developing breast cancer is nearly the same in the affected and unaffected breast (side) indicating only coincidence of risk factors. For atypical lobular hyperplasia there is high incidence of ipsilateral breast cancers indicating a possible direct carcinogenetic link.

About one percent of breast cancer develops in males. It is estimated that about 2,140 new cases are diagnosed annually in the United States (US) and about 300 in the United Kingdom (UK). The number of annual deaths in the US is about 440 (for 2016 "but fairly stable over the last 30 years"). In a study from India, eight out of 1,200 (0.7%) male cancer diagnoses in a pathology review represented breast cancer. Incidence of male breast cancer has been increasing which raises the probability of other family members developing the disease. The relative risk of breast cancer for a female with an affected brother is approximately 30% higher than for a female with an affected sister. The tumor can occur over a wide age range, but typically appears in males in their sixties and seventies.

Known risk factors include radiation exposure, exposure to female hormones (estrogen), and genetic factors. High estrogen exposure may occur by medications, obesity, or liver disease, and genetic links include a high prevalence of female breast cancer in close relatives. Chronic alcoholism has been linked to male breast cancer. The highest risk for male breast cancer is carried by males with Klinefelter syndrome. Male BRCA mutation carriers are thought to be at higher risk for breast cancer as well, with roughly 10% of male breast cancer cases carrying BRCA2 mutations, and BRCA1 mutation being in the minority.

The specific causes of DCIS are still unknown. The risk factors for developing this condition are similar to those for invasive breast cancer.

Some women are however more prone than others to developing DCIS. Women considered at higher risks are those who have a family history of breast cancer, those who have had their periods at an early age or who have had a late menopause. Also, women who have never had children or had them late in life are also more likely to get this condition.

Long-term use of estrogen-progestin hormone replacement therapy (HRT) for more than five years after menopause, genetic mutations (BRCA1 or BRCA2 genes), atypical hyperplasia, as well as radiation exposure or exposure to certain chemicals may also contribute in the development of the condition. Nonetheless, the risk of developing noninvasive cancer increases with age and it is higher in women older than 45 years.

Triple-negative breast cancer accounts for approximately 15%-25% of all breast cancer cases. The overall proportion of TNBC is very similar in all age groups. Younger women have a higher rate of basal or BRCA related TNBC while older women have a higher proportion of apocrine, normal-like and rare subtypes including neuroendocrine TNBC.

Among younger women, African American and Hispanic women have a higher risk of TNBC, with African Americans facing worse prognosis than other ethnic groups.

In 2009, a case-control study of 187 triple-negative breast cancer patients described a 2.5 increased risk for triple-negative breast cancer in women who used oral contraceptives (OCs) for more than one year compared to women who used OCs for less than one year or never. The increased risk for triple-negative breast cancer was 4.2 among women 40 years of age or younger who used OCs for more than one year, while there was no increased risk for women between the ages of 41 and 45. Also, as duration of OC use increased, triple-negative breast cancer risk increased.

Most patients diagnosed with Paget's disease of the nipple are over age 50, but rare cases have been diagnosed in patients in their 20s. The average age at diagnosis is 62 for women and 69 for men. The disease is rare among both women and men.

Among women worldwide, breast cancer is the most common cause of cancer death. Breast self-examination (BSE) is an easy but unreliable method for finding possible breast cancer. Factors that appear to be implicated in decreasing the risk of, early diagnosis of. or recurrence of breast cancer are regular breast examinations by health care professionals, regular mammograms, self-examination of breasts, healthy diet, and exercise to decrease excess body fat.

LCIS (lobular neoplasia is considered pre-cancerous) is an indicator (marker) identifying women with an increased risk of developing invasive breast cancer. This risk extends more than 20 years. Most of the risk relates to subsequent invasive ductal carcinoma rather than to invasive lobular carcinoma.

While older studies have shown that the increased risk is equal for both breasts, a more recent study suggests that the ipsilateral (same side) breast may be at greater risk.

The presence of three factors for the prognosis has been suggested, whether there is a palpable mass of the disease, whether lymph nodes are positive and whether there is an underlying malignant cancer.

If there is none of these, the five- and 10-year survival is 85% and 80% respectively, with adjuvant chemotherapy even 95% and 90%. If there is a palpable mass, it is 32% and 31% respectively, with adjuvant chemotherapy (40% and 35%).

Positive lymph-nodes have been positively associated with a palpable mass and affect the prognosis to be now just 28% survival after 10 years (vs 79% without palpable mass and without affected lymph-nodes). Involvement of the lymph nodes does not directly cause any harm, but is merely an indicator of systemic spread.

Furthermore, patients with an identifiable associated underlying breast tumor have a survival rate of 38-40% at five years and a survival rate of 22-33% at 10 years. The death rate of metastatic breast carcinoma in patients with mammary Paget's disease and underlying cancer is 61.3%, with a 10-year cumulative survival rate of 33%.

80% of cases in the United States are diagnosed by mammography screening.

Adjusted for age and stage the prognosis for breast cancer in males is similar to that in females. Prognostically favorable are smaller tumor size and absence or paucity of local lymph node involvement. Hormonal treatment may be associated with hot flashes and impotence.

Age distribution and relation to breastfeeding duration is suggestive of some sort of involvement of hormones in the aetiology, however significant differences exist compared to normal breast cancer.

Typically IBC shows low levels of estrogen and progesterone receptor sensitivity, corresponding with poor outcome. In cases with positive estrogen receptor status antihormonal treatment is believed to improve outcome.

Paradoxically some findings suggest that especially aggressive phenotypes of IBC are characterised by high level of NF kappaB target gene expression which can be - under laboratory conditions - successfully modulated by estrogen, but not by tamoxifen.

There are usually no adverse side effects to this condition. In almost all cases it subsides after menopause. A possible complication arises through the fact that cancerous tumors may be more difficult to detect in women with fibrocystic changes.

Breast diseases can be classified either with disorders of the integument, or disorders of the reproductive system. A majority of breast diseases are noncancerous.

One known cause of triple negative breast cancer is germline mutations. These are alterations within the heritable lineage that is being passed down to the offspring. 15% of TNBC can be traced back to germline mutations that are within the BRCA1 and BRCA2 genes (Song 2014). These genes were identified as high risk for triple negative due to their high predisposition for cancers of the breasts, ovaries, pancreas, and prostate (Pruss 2014). Changes or mutations in 19p13.1 and MDM4 loci have also been associated with triple negative breast cancer, but not other forms of breast cancer, thus triple negative tumors may be distinguished from other breast cancer subtypes by a unique pattern of common and rare germline alterations (Kristen 2013).

Fibrosclerosis of the breast is most frequently used to mean sclerosing lobular adenosis of the breast. It is not well known if it is related to non-sclerosing adenosis of the breast (which is normally classified as fibrocystic breast changes) and unlike this it is believed to be correlated with an elevated risk of breast cancer.

It is estimated that 7% of women in the western world develop palpable breast cysts.

There is preliminary evidence that women with breast cysts may be at an increased risk of breast cancer, especially at younger ages.

In males, the occurrence of breast cysts is rare and may (but need not) be an indication of malignancy.

LCIS may be treated with close clinical follow-up and mammographic screening, tamoxifen or related hormone controlling drugs to reduce the risk of developing cancer, or bilateral prophylactic mastectomy. Some surgeons consider bilateral prophylactic mastectomy to be overly aggressive treatment except for certain high-risk cases.

A number of genes are associated with HBOC. The most common of the known causes of HBOC are:

- BRCA mutations: Harmful mutations in the "BRCA1" and "BRCA2" genes can produce very high rates of breast and ovarian cancer, as well as increased rates of other cancers.

Other identified genes include:

- "TP53": Mutations cause Li-Fraumeni syndrome. It produces particularly high rates of breast cancer among younger women with mutated genes, and despite being rare, 4% of women with breast cancer under age 30 have a mutation in this gene.

- "PTEN": Mutations cause Cowden syndrome, which produces hamartomas (benign polyps) in the colon, skin growths, and other clinical signs, as well as an increased risk for many cancers.

- "CDH1": Mutations are associated with lobular breast cancer and gastric cancer.

- "STK11": Mutations produce Peutz–Jeghers syndrome. It is extremely rare, and creates a predisposition to breast cancer, intestinal cancer, and pancreatic cancer.

- "CHEK2": Approximately one out of 40 northern Europeans have a mutation in this gene, making it a common mutation. Considered a moderate-risk mutation, it may double or triple the carrier's lifetime risk of breast cancer, and also increase the risk of colon cancer and prostate cancer.

- "ATM": Mutations cause ataxia telangectasia; female carriers have approximately double the normal risk of developing breast cancer.

- "PALB2": Studies vary in their estimate of the risk from mutations in this gene. It may be moderate risk, or as high as "BRCA2".

Approximately 45% of HBOC cases involve unidentified genes, or multiple genes.

The relative risk of breast cancer based on a median follow-up of 8 years, in a case control study of US registered nurses, is 3.7.

Some women who have pain in one or both breasts may fear breast cancer. However, breast pain is not a common symptom of cancer. The great majority of breast cancer cases do not present with symptoms of pain, though breast pain in older women is more likely to be associated with cancer.