Spironolactone is a competitive antagonist of the aldosterone receptor. Hypokalemia and hypeension in patients with primary aldosteronism can be controlled by spironolactone, 50-100 mg/d. Although spironolactone is an effective aldosterone receptor antagonist, it is not without side effects that can limit its use in the chronic treatment of this disease. The most impoant are anti-androgenic reactions. Spironolactone acts as an antiandrogen by decreasing the production of testosterone by the adrenal gland and by preventing DHT (dihydrotestosterone) from binding to its androgen receptor. As a result of this, in the long run, gynecomastia occurs in more than 10% of the treated men. Impotence, loss of libido, and menstrual irregularities are also common side effects of spironolactone therapy. On the other side, these spironolactone features are the basis for its usage in the treatment of hirsutism, acne, and alopecia. Spironolactone is also used by transsexuals in the feminizing regimen because of its anti-androgenic actions. Cardiac arrhythmia is not a frequent adverse reaction to spironolactone treatment, and it is seen only in the presence of significant hyperkalemia. Dehydration is usually very mild, and can be prevented with adequate water intake. Hyperkalemia develops in 5-10% of treated patients, especially if renal function is compromised, or the patient is diabetic, or elderly. Also Know: Spironolactone (Aldactone) is a direct aldosterone receptor antagonist in collecting tubules. It acts to inhibit aldosterone-mediated Na+ reabsorption and K+ secretion. Ref: Morgan, Jr. G.E., Mikhail M.S., Murray M.J. (2006). Chapter 31. Renal Physiology & Anesthesia. In G.E. Morgan, Jr., M.S. Mikhail, M.J. Murray (Eds), Clinical Anesthesiology, 4e