(2) Chlorothiazide (Diuril). This drug is used as a diuretic and as an
antihypertensive. It is available in both parenteral and oral dosage forms. For side
effects, refer to hydrochlorothiazide.
(3) Chlorthalidone (Hygroton). Although chlorthalidone is
not the same chemically as the thiazide diuretics, it has the same effects as these
agents. For indications and side effects, you should refer to hydrochlorothiazide.
c. Potassium-Sparing Diuretics. This type of diuretic is used when there is a
need to maintain normal levels of potassium in the patient along with the diuresis. The
specific mechanisms of actions of selected drugs in this category.
(1) Spironolactone (Aldactone). Spironolactone causes sodium
diuresis and potassium retention by acting as an aldosterone competitive antagonist.
That is, this drug acts on the distal tubule to block the sodium-potassium exchange
mechanism. The net result is sodium loss and potassium retention. Consequently, by
antagonizing aldosterone, sodium as well as water diuresis and potassium retention are
affected. Spironolactone is used for primary hyperaldosteronism, edema associated
with congestive heart failure, cirrhosis of the liver or ascites, essential hypertension, and
in hypokalemia when other means are considered inappropriate or inadequate. The
usual dose of this drug is from 25 to 400 milligrams per day depending upon the
condition of the patient. Although spironolactone is a mild diuretic, it can hasten major
side effects such as gastrointestinal symptoms (for example: cramping and diarrhea),
lethargy, hyperkalemia, and hyponatremia. Hyperkalemia is a major side effect that
occurs in patients who have impaired renal function. Hyperkalemia can cause
irregularities that may be fatal. Spirolactone also causes estrogen-like side effects
because of its hormone-like structure.
(2) Triamterene (Dyrenium). While triamterene produces effects similar to
those of spironolactone, the effects produced by triamterene are not dependent on the
presence of aldosterone. This agent acts directly on the distal tubule where it prevents
the passage of sodium across the membrane of the tubule. Thus, by blocking sodium
reabsorption, potassium loss is reduced. Triamterene is used for edema associated
with congestive heart failure and cirrhosis of the liver. The usual dosage of this drug is
from 25 to 200 milligrams per day. The daily dose should not exceed 300 milligrams.
Side effects associated with this agent include electrolyte imbalances, hyperkalemia,
weakness, and dry mouth. Like spironolactone, hyperkalemia is a major side effect
which can occur in patients who have impaired renal function or when the drug is
d. Carbonic Anhydrase Inhibitor Diuretics. Carbonic anhydrase inhibitors
produce diuresis by inhibiting carbonic anhydrase in the renal tubules. Carbonic
anhydrase is an enzyme that catalyzes the following reaction: