In newborns, gynecomastia is caused by estrogen from the mother. Breast buds are common in baby boys. Breast buds tend to go away gradually by 6 months of age, but they can last longer in some babies.

In preteen boys, breast buds are common during puberty. The buds may last up to 2 years, but they tend to go away within the first year. Gynecomastia can also be caused by an estrogen-producing tumor.

In teen boys, gynecomastia is caused by the hormonal changes of puberty. Gynecomastia occurs in many boys during early puberty to middle puberty. It usually goes away within 6 months to 2 years.

In adult males, gynecomastia is usually caused by another condition, such as liver or lung cancer, cirrhosis of the liver, overactive thyroid, or by hormone problems, such as cancer of the pituitary gland, adrenal glands, or testicles. Alcohol, marijuana, methamphetamine, and heroin use also may cause gynecomastia. In older males, gynecomastia can be caused by a change in hormone levels.

Use of certain medicines may also cause gynecomastia, including:

• Steroids, such as prednisone or dexamethasone.
• Medicines used to treat ulcers (such as cimetidine).
• Medicines used to treat epilepsy (such as phenytoin [Dilantin]).
• Digitalis and other heart medicines.
• Chemotherapy drugs, especially alkylating agents, a family of anticancer drugs that interfere with cell DNA and inhibit cancer cell growth
• Antiandrogen drugs (such as flutamide, cyproterone, and spironolactone).
• Antianxiety and antidepressant medicines (such as diazepam [Valium] and tricyclic antidepressants).