Causes of Gynecomastia.

The following medical conditions are some of the possible causes of Gynecomastia. There are likely to be other possible causes.

• Klinefelter syndrome - A genetic condition where males have at least one extra X chromosome or extra copies of both the X and Y chromosomes in each cell. Normally male cells contain one X and one Y chromosome in each cell. The condition is not inherited but is a result of problems during cell division. Klinefelter syndrome variants is a more severe form of the condition as it involves more than one extra X or X and Y chromosome in each cell.
• Seminiferous tubule dysgenesis
• Testicular failure

• Androgen-insensitivity syndrome -  Females with male XY genetics but inability to respond to testosterone. Androgen, also called androgenic hormones or testoids, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes the activity of the accessory male sex organs and development of male secondary sex characteristics. 

• Androgen receptor defects
• Biosynthetic defects in testosterone production
• Feminizing adrenocortical tumors
• Isolated Leutenizing hormone deficiency
• Testicular germ cell tumor
• Leydig cell tumor
• Sertoli cell tumor
• Pseudogynecomastia
• Obesity - may cause similar enlarged appearance.
• Reifenstein syndrome
• Kallmann syndrome
• Liver cirrhosis
• Puberty - all adolescent males experience some male breast enlargment; it is often temporary.

Another reasons are the effects of following drugs.

• Antiandrogens (flutamide, finasteride, ketoconazole)
• Gastrointestinal agents (cimetidine, ranitidine)
• Digoxin
• Calcium channel blockers (verapamil, nifedipine)
• Marijuana
• Heroin
• Methadone
• Amphetamines
• Androgens
• Anabolic steroids
• Estrogens
• Phenothiazines
• Diazepam
• Tricyclic antidepressants
• Hyperthyroidism
• Klinefelter's syndrome
• Isoniazid
• Spirionolactone
• Cimetidine
• Omeprazole
• Oestrogen
• Methyldopa

Other reasons that might cause unnatural swelling of breast among male.

• Physiological
• Testicular tumours
• Adrenal carcinoma
• Breast cancer
• Hepatic carcinoma
• Hypothyroidism
• Pituitary tumor