Gynecomastia is the excessive growth or enlargement of breast tissue in males. As breast development begins, one breast may grow more than the other. Gynecomastia typically occurs after hormonal changes during adolescence. However, it can also be noticeable in young children and very elderly individuals.

Most cases of gynecomastia are benign (non-cancerous). Gynecomastia can be associated with changes in hormone levels.

Changes in the amounts of male hormone (testosterone) and female hormone (estrogen) often lead to gynecomastia.

Additionally, many medications, including antidepressants, antibiotics, chemotherapy, prostate cancer treatments, ulcer and cardiovascular medications, can cause gynecomastia as a side effect. Gynecomastia is also known to be caused by the use of illegal substances such as anabolic steroids, heroin, or marijuana.

Some diseases and medical conditions can also cause gynecomastia.

Some of these include:
- Congenital disorders (birth defects)
- Thyroid disorders
- Injury or trauma
- Obesity
- Liver diseases
- Kidney disease
- Lung cancer
- Testicular cancer
- Adrenal gland or pituitary gland tumors
- Gynecomastia can be seen in newborns for a short time. This often occurs because the estrogen transferred from the mother persists in the baby's circulation for a while after birth.

Breast cancer and gynecomastia are not related. Breast cancer in men is quite rare. However, your doctor may want to perform some tests to rule out breast cancer.