Gynecomastia is the development of abnormally large breasts in males. It is related to the excess growth of breast tissue, rather than excess fat tissue.

The condition may occur in one or both breasts and begins as a small lump beneath the nipple, which may be tender. The breasts often enlarge unevenly. Gynecomastia during puberty is not uncommon and usually goes away over a period of months.

In newborns, breast development may be associated with milk flow (galactorrhea). This condition usually lasts for a couple of weeks, but in rare cases may last until the child is 2 years old.

Androgens are hormones that create male characteristics, such as hair growth, muscle size, and a deep voice. Estrogens are hormones that create female characteristics. All men have both androgens and estrogens.

Changes in the levels of these hormones, or in how the body uses or responds to these hormones can cause enlarged breasts in men.

More than half of boys develop gynecomastia during puberty.

Other causes include:

• Aging
• Cancer chemotherapy
• Chronic liver disease
• Exposure to anabolic steroid hormones
• Exposure to estrogen hormone
• Kidney failure and dialysis
• Marijuana use
• Hormone treatment for prostate cancer
• Radiation treatment of the testicles
• Side effects of some medications (ketoconazole, spironolactone, metronidazole, cimetidine (Tagamet))
• Testosterone (male hormone) deficiency

Rare causes include:

• Genetic defects
• Overactive thyroid
• Tumors

Breast cancer in men is rare. Signs that may suggest breast cancer include:

• One-sided breast growth
• Firm or hard breast lump that feels like it is attached to the tissue
• Skin ulcer over the breast
• Bloody discharge from the nipple