There are case reports of gigantomastia occurring in infants as well.

The underlying cause of the rapidly growing breast connective tissue, resulting in gigantic proportions, has not been well-elucidated. However, proposed factors have included increased levels/expression of or heightened sensitivity to certain hormones (e.g., estrogen, progesterone, and prolactin) and/or growth factors (e.g., hepatic growth factor, insulin-like growth factor 1, and epidermal growth factor) in the breasts. Macromastic breasts are reported to be composed mainly of adipose and fibrous tissue, while glandular tissue remains essentially stable.

Macromastia occurs in approximately half of women with aromatase excess syndrome (a condition of hyperestrogenism). Hyperprolactinemia has been reported as a cause of some cases of macromastia. Macromastia has also been associated with hypercalcemia (which is thought to be due to excessive production of parathyroid hormone-related protein) and, rarely, systemic lupus erythematosus and pseudoangiomatous stromal hyperplasia. It is also notable that approximately two-thirds of women with macromastia are obese. Aside from aromatase (as in aromatase excess syndrome), at least two other genetic mutations (one in PTEN) have been implicated in causing macromastia.

A handful of drugs have been associated with gigantomastia, including penicillamine, bucillamine, neothetazone, ciclosporin, and indinavir.

In Rinker's study, 55% of respondents reported an adverse change in breast shape after pregnancy. Many women mistakenly attribute the changes and their sagging breasts to breastfeeding, and as a result some are reluctant to nurse their infants. Research shows that breastfeeding is not the factor that many thought it was. Rinker concluded that "Expectant mothers should be reassured that breastfeeding does not appear to have an adverse effect upon breast appearance." Also discounted as causes affecting ptosis are weight gain during pregnancy and lack of participation in regular upper body exercise.

Approximately 10–25 percent of cases are estimated to result from the use of medications. This is known as non-physiologic gynecomastia. Medications known to cause gynecomastia include ketoconazole, cimetidine, gonadotropin-releasing hormone analogues, human growth hormone, human chorionic gonadotropin, 5α-Reductase inhibitors such as finasteride and dutasteride, estrogens such as those used in transgender women and men with prostate cancer, and antiandrogens such as bicalutamide, flutamide, and spironolactone. Medications that are probably associated with gynecomastia include calcium channel blockers such as verapamil, amlodipine, and nifedipine; risperidone, olanzapine, anabolic steroids, alcohol, opioids, efavirenz, alkylating agents, and omeprazole. Certain components of personal care products such as lavender or tea tree oil and certain supplements such as dong quai and "Tribulus terrestris" have been associated with gynecomastia.

According to Rinker's research, there are several key factors. A history of cigarette smoking "breaks down a protein in the skin called elastin, which gives youthful skin its elastic appearance and supports the breast." The number of pregnancies was strongly correlated with ptosis, with the effects increasing with each pregnancy. As most women age, breasts naturally yield to gravity and tend to sag and fold over the inframammary crease, the lower attachment point to the chest wall. This is more true for larger-breasted women. The fourth reason was significant weight gain or loss (greater than ). Other significant factors were higher body mass index and larger bra cup size.

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.

Gynecomastia is thought to be caused by an altered ratio of estrogens to androgens mediated by an increase in estrogen production, a decrease in androgen production, or a combination of these two factors. Estrogen acts as a growth hormone to increase the size of male breast tissue. The cause of gynecomastia is unknown in around 25% of cases. Drugs are estimated to cause 10–25% of cases of gynecomastia.

Certain health problems in men such as liver disease, kidney failure or low testosterone can cause breast growth in men. Drugs and liver disease are the most common cause in adults. Other medications such as methadone, aldosterone antagonists (spironolactone and epelerenone), HIV medication, cancer chemotherapy, hormone treatment for prostate cancer, heartburn and ulcer medications, calcium channel blockers, antifungal medications such as ketoconazole, antibiotics such as metronidazole, tricyclic antidepressants such as amitriptyline, herbals such as lavender, tea tree oil, and dong quai are also known to cause gynecomastia. Phenothrin, an insecticide, possesses antiandrogen activity, and has been associated with gynecomastia.

The estimated figures for the prevalence of fibrocystic breast changes in women over lifetime vary widely in the literature, with estimates ranging from about 30 to 60 % over about 50 to 60 % to about 60 to 75% of all women.

The condition is most common among women between 30 and 50 years of age.

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.

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.

Breast atrophy is the normal or spontaneous atrophy or shrinkage of the breasts.

Breast atrophy commonly occurs in women during menopause when estrogen levels decrease. It can also be caused by hypoestrogenism and/or hyperandrogenism in women in general, such as in antiestrogen treatment for breast cancer, in polycystic ovary syndrome (PCOS), and in malnutrition such as that associated with eating disorders like anorexia nervosa or with chronic disease. It can also be an effect of weight loss.

In the treatment of gynecomastia in males and macromastia in women, and in hormone replacement therapy (HRT) for trans men, breast atrophy may be a desired effect.

Examples of treatment options for breast atrophy, depending on the situation/when appropriate, can include estrogens, antiandrogens, and proper nutrition or weight gain.

Any deformity of the breasts is only apparent during puberty and this may lead to psychosexual problems with girls in very early puberty being affected psychologically due to the unusual shape of the breast. Surgical papers about the techniques useful in correcting tubular breasts note that

even when results are not perfect, the psychological impact of treatment is immense, with notable improvements in self-esteem to the level where the person engages in normal social activities.

The appearance of tuberous breasts can potentially be changed through surgical procedures, including the tissue expansion method and breast implants.

The procedure to change the appearance of tuberous breasts can be more complicated than a regular breast augmentation, and some plastic surgeons have specialist training in tuberous breast correction. As tuberous breasts are a congenital deformity, referral for treatment under the National Health Service may be possible in the United Kingdom. A starting point for those seeking such a referral may be a visit to their local General Practitioner. For those seeking non-surgical solutions, counseling may be recommended as a way of coming to terms with body image.

Cyclical breast pain (cyclical mastalgia) is often associated with fibrocystic breast changes or duct ectasia and thought to be caused by changes of prolactin response to thyrotropin. Some degree of cyclical breast tenderness is normal in the menstrual cycle, and is usually associated with menstruation and/or premenstrual syndrome (PMS).

Noncyclical breast pain has various causes and is harder to diagnose. Noncyclical pain has frequently its root cause outside the breast. Some degree of non-cyclical breast tenderness can normally be present due to hormonal changes in puberty (both in girls and boys), in menopause and during pregnancy. After pregnancy, breast pain can be caused by breastfeeding. Other causes of non-cyclical breast pain include alcoholism with liver damage (likely due to abnormal steroid metabolism), mastitis and medications such as digitalis, methyldopa (an antihypertensive), spironolactone, certain diuretics, oxymetholone (an anabolic steroid), and chlorpromazine (a typical antipsychotic). Also, shingles can cause a painful blistering rash on the skin of the breasts.

Failure to remove breast milk, especially in the first few days after delivery when the milk comes in and fills the breast, and at the same time blood flow to the breasts increases, causing congestion. The common reasons why milk is not removed adequately are delayed initiation of breastfeeding, infrequent feeds, poor attachment, ineffective suckling., a sudden change in breastfeeding routine, suddenly stopping breastfeeding, or if your baby suddenly starts breastfeeding less than usual.

The development of breast cysts may be prevented to some degree, according to the majority of the specialists. The recommended measures one is able to take in order to avoid the formation of the cysts include practicing good health and avoiding certain medications, eating a balanced diet, taking necessary vitamins and supplements, getting exercise, and avoiding stress.

Although caffeine consumption does not have a scientifically proved connection with the process of cyst development, many women claim that their symptoms are relieved if avoiding it. Some doctors recommend reducing the amount of caffeine in one's diet in terms of both beverages and foods (such as chocolate). Also reducing salt intake may help in alleviating the symptoms of breast cysts, although, again, there is no scientific linkage between these two. Excessive sugar consumption as well as undetected food allergies, such as to gluten or lactose, may also contribute to cyst development.

The mother must remove the breast milk. If the baby can attach well and suckle, then she should breastfeed as frequently as the baby is willing. If the baby is not able to attach and suckle effectively, she should express her milk by hand or with a pump a few times until the breasts are softer, so that the baby can attach better, and then get them to breastfeed frequently.

She can apply warm compresses to the breast or take a warm shower before expressing, which helps the milk to flow. She can use cold compresses after feeding or expressing, which helps to reduce the oedema.

Engorgement occurs less often in baby-friendly hospitals which practise the Ten Steps and which help mothers to start breastfeeding soon after delivery.

Regular breastfeeding can be continued. The treatment for breast engorgement can be divided into non-medical and medical methods. The non-medical methods include hot/cold packs, Gua-Sha (scraping therapy), acupuncture and cabbage leaves whereas medical methods are proteolytic enzymes such as serrapeptase, protease, and subcutaneous oxytocin. Evidence from published clinical trials on the effectiveness of treatment options is of weak quality and is not strong enough to justify a clinical recommendation.

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.

A meta analysis of cohort studies of alcohol consumption and breast cancer mortality showed no association between alcohol consumption before or after breast cancer diagnosis and recurrence after treatment.

The procedure to remedy micromastia is breast enlargement, most commonly augmentation mammoplasty using breast implants. Other techniques available involve using muscle flap-based reconstructive surgery techniques (latissimus dorsi and rectus abdominus muscles), microsurgical reconstruction, or fat grafting.

Another potential treatment is hormonal breast enhancement, such as with estrogens.

Micromastia (also called hypomastia, breast aplasia, breast hypoplasia, or mammary hypoplasia) is a medical term describing the postpubertal underdevelopment of a woman's breast tissue. Just as it is impossible to define 'normal' breast size, there is no objective definition of micromastia. Breast development is commonly asymmetric and one or both breasts may be small. This condition may be a congenital defect associated with underlying abnormalities of the pectoral muscle (as in Poland's syndrome), related to trauma (typically surgery or radiotherapy) or it may be a more subjective aesthetic description.

Self perceived micromastia involves a discrepancy between a person's body image, and her internalized images of appropriate or desirable breast size and shape. Societal ideals over breast size vary over time, but there exist many conceived ideas involving breasts and sexual attractiveness and identity across different cultures.

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.

An estrogen-dependent condition, disease, disorder, or syndrome, is a medical condition that is, in part or full, dependent on, or is sensitive to, the presence of estrogenic activity in the body.

Known estrogen-dependent conditions include mastodynia (breast pain/tenderness), breast fibroids, mammoplasia (breast enlargement), macromastia (breast hypertrophy), gynecomastia, breast cancer, precocious puberty in girls, melasma, menorrhagia, endometriosis, endometrial hyperplasia, adenomyosis, uterine fibroids, uterine cancers (e.g., endometrial cancer), ovarian cancer, and hyperestrogenism in males such as in certain conditions like cirrhosis and Klinefelter's syndrome.

Such conditions may be treated with drugs with antiestrogen actions, including selective estrogen receptor modulators (SERMs) such as tamoxifen and clomifene, estrogen receptor antagonists such as fulvestrant, aromatase inhibitors such as anastrozole and exemestane, gonadotropin-releasing hormone (GnRH) analogues such as leuprolide and cetrorelix, and/or other antigonadotropins such as danazol, gestrinone, megestrol acetate, and medroxyprogesterone acetate.

Nipple discharge refers to any fluid that seeps out of the nipple of the breast. Discharge from the nipple does not occur in lactating women. And discharge in non-pregnant women or women who are not breasfeeding may not cause concern. Men that have discharge from their nipples are not typical. Discharge from the nipples of men or boys may indicate a problem. Discharge from the nipples can appear without squeezing or may only be noticeable if the nipples are squeezed. One nipple can have discharge while the other does not. The discharge can be clear, green, bloody, brown or straw-colored. The consistenct can be thick, thin, sticky or watery.

Some cases of nipple discharge will clear on their own without treatment. Nipple discharge is most often not cancer (benign), but rarely, it can be a sign of breast cancer. It is important to find out what is causing it and to get treatment. Here are some reasons for nipple discharge:

- Pregnancy

- Recent breastfeeding

- Rubbing on the area from a bra or t-shirt

- Trauma

- Infection

- Inflammation and clogging of the breast ducts

- Noncancerous pituitary tumors

- Small growth in the breast that is usually not cancer

- Severe underactive thyroid gland (hypothyroidism)

- Fibrocystic breast (normal lumpiness in the breast)

- Use of certain medicines

- Use of certain herbs, such as anise and fennel

- Widening of the milk ducts

- Intraductal pipilloma

- Subareolar abscess

- Mammary duct ectasia

- Pituitary tumor

Sometimes, babies can have nipple discharge. This is caused by hormones from the mother before birth. It usually goes away in 2 weeks. Cancers such as Paget disease (a rare type of cancer involving the skin of the nipple) can also cause nipple discharge.

Nipple discharge that is NOT normal is bloody, comes from only one nipple, or comes out on its own without squeezing or touching the nipple. Nipple discharge is more likely to be normal if it comes out of both nipples or happens when the nipple is squeezed. Squeezing the nipple to check for discharge can make it worse. Leaving the nipple alone may make the discharge stop.

Nipple discharge may be a symptom of breast cancer or a pituitary tumor. Skin changes around the nipple may be caused by Paget disease.