Normal hormone changes are the most common cause of breast development in newborns, boys, and men. There are other causes as well.

HORMONE CHANGES

Breast enlargement is usually caused by an imbalance of estrogen (female hormone) and testosterone (male hormone). Males have both types of hormones in their body. Changes in the levels of these hormones, or in how the body uses or responds to these hormones, can cause enlarged breasts in males.

In newborns, breast growth is caused by being exposed to estrogen from the mother. About one half of boy babies are born with enlarged breasts, called breast buds. They usually go away in 2 to 6 months, but can last longer.

In preteens and teens, breast growth is caused by normal hormone changes that occur in puberty. More than one half of boys develop some breast enlargement during puberty. Breast growth often goes away in about 6 months to 2 years.

In men, hormone changes due to aging can cause breast growth. This may occur more often in overweight or obese men and in men age 50 and older.

HEALTH CONDITIONS

Certain health problems can cause breast growth in adult men, including:

Chronic liver disease

Kidney failure and dialysis

Low testosterone level

Obesity

Rare causes include:

Genetic defects

Overactive thyroid or underactive thyroid

Tumors (including benign tumor of the pituitary gland, called prolactinoma)

MEDICINES AND MEDICAL TREATMENT

Some medicines and treatments that can cause breast growth in men include:

Cancer chemotherapy

Hormone treatment for prostate cancer, such as flutamide (Proscar), or for enlarged prostate, such as finasteride (Propecia) or bicalutamide

Radiation treatment of the testicles

HIV/AIDS medicines

Corticosteroids and anabolic steroids

Estrogen (including those in soy products)

Heartburn and ulcer medicines, such as cimetidine (Tagamet) or proton pump inhibitors

Anti-anxiety medicines, such as diazepam (Valium)

Heart medicines, such as spironolactone (Aldactone), digoxin (Lanoxin), amiodarone, and calcium channel blockers

Antifungal medicines, such as ketoconazole (Nizoral)

Antibiotics such as metronidazole (Flagyl)

Tricyclic antidepressants such as amitriptyline (Elavil)

Herbals such as lavender, tea tree oil, and dong quai

Opioids

DRUG AND ALCOHOL USE

Using certain substances can cause breast enlargement:

Alcohol

Amphetamines

Heroin

Marijuana

Methadone

Men who have enlarged breasts may have an increased risk for breast cancer. Breast cancer in men is rare. Signs that may suggest breast cancer include: