Is it safe to take water pill while breastfeeding?

Depends on which one: Hydrochlorothiazide, triamterene, & Amiloride do not cause birth defects per some studies. Older studies raised concerns that thiazide diuretics might cause thrombocytopenia, but subsequent studies have shown that there is no decrease in platelets. Altho diuretics do affect the plasma volume expansion of normal pregnancy, this has not been correlated with a negative effect on fetal growth. [nih].