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Gynecomastia is a benign enlargement of the male breast resulting from a proliferation of the glandular component of the breast. Gynecomastia is defined clinically by the presence of a rubbery or firm mass extending concentrically from the nipples. Although the condition is usually bilateral, it can be unilateral. The condition known as pseudogynecomastia, or lipomastia, is characterized by fat deposition without glandular proliferation. (See Etiology, Presentation, and Workup.)

Various drugs are implicated in gynecomastia and can be classified into the following categories (although drugs in the same class do not all cause gynecomastia to the same extent)[7, 8] :

•Estrogens or drugs with estrogenlike activity - Include diethylstilbestrol, digitalis, and phytoestrogens, as well as estrogen-contaminated food and estrogen-containing cosmetics
•Drugs that enhance estrogen synthesis - Include gonadotropins, clomiphene, phenytoin, and exogenous testosterone
•Drugs that inhibit testosterone synthesis or action - Include ketoconazole, metronidazole, alkylating agents, cisplatin, spironolactone, cimetidine, flutamide, finasteride, and etomidate
•Drugs that act by unknown mechanisms - Include isonicotinic acid hydrazide, methyldopa, busulfan, tricyclic antidepressants, diazepam, penicillamine, omeprazole, phenothiazines, calcium channel blockers, angiotensin-converting enzyme (ACE) inhibitors, alcohol, marijuana, and heroin