Which medication is most likely to cause gynecomastia in a male patient?

Spironolactone is recognized for its potential to cause gynecomastia in male patients. This effect is primarily due to its action as an aldosterone antagonist, which can lead to an increase in estrogen levels relative to testosterone in the body. Spironolactone has a structural similarity to steroids and can bind to androgen receptors, which may disrupt the normal hormonal balance and lead to breast tissue growth.

The mechanism involves the inhibition of testosterone synthesis and increased conversion to estrogen, contributing to the development of gynecomastia. This side effect is particularly noted with long-term use or higher doses of spironolactone.

In contrast, furosemide and hydrochlorothiazide are diuretics that are less likely to impact estrogen levels and do not have the same gender-specific side effects as spironolactone. Triamterene, being a potassium-sparing diuretic, also lacks the hormonal activity associated with gynecomastia. Therefore, spironolactone stands out as the medication most closely linked with this particular adverse effect.