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Q. I know that many men, including me, develop larger breasts as they age. But my right breast appears larger than the left one. Should I be worried?

A. Indeed, breast enlargement in men has become more common. The two predominant reasons are excess weight and a condition called gynecomastia (pronounced guy-nuh-co-MAS-tee-uh).

Men gain weight because of fat deposition in several locations: under the skin (subcutaneous fat), inside the abdominal cavity (visceral fat), and in the breasts. Breast enlargement from fat buildup is usually symmetrical, with an equal amount in both breasts. Fat is spread throughout the whole breast area. Gynecomastia refers to an enlargement of the breast's glandular tissue, not by excess fat. The glandular tissue is concentrated under the nipple.

Male glandular tissue is influenced by the balance of androgens (the group of male sex hormones, with testosterone being the major one), which keep breast tissue small, and female hormones like estrogen, which stimulate breast tissue growth. Most people produce both hormones. In men, the androgens are dominant, while estrogens have the upper hand in women. But even a slight shift in the balance between testosterone and estrogen can lead to gynecomastia.

With gynecomastia, the breasts have a firm or rubbery consistency. They can be tender, compared with the softer feel from excess fat that does not cause physical discomfort. While gynecomastia usually occurs in both breasts, it can affect one breast more than the other, leading to size differences.

Gynecomastia has been reported as a potential side effect of many drugs. Common ones include

• finasteride (Proscar) and dutasteride (Avodart), used to treat prostate enlargement
• digoxin (Lanoxin), used for heart disease
• spironolactone (Aldactone), a diuretic
• cimetidine, used for stomach inflammation and heartburn
• ketoconazole, used for fungal infections.

In older men, levels of testosterone can fall enough to cause gynecomastia. Also, men undergoing hormonal treatment for prostate cancer to lower testosterone will develop a balance favoring the estrogen effects on breast tissue. Liver disease is another reason for an imbalance favoring estrogen.

Asymmetric breast enlargement always warrants a medical evaluation. Your doctor will want to determine whether this is more likely due to fat accumulation or gynecomastia. The doctor also will examine your breasts for any lumps. Although breast cancer in men is rare, an estimated 2,800 cases in men are diagnosed annually.

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