What is Lasix (furosemide)?

What is Lasix (Furosemide)?

Lasix (furosemide) is a potent loop diuretic that acts on the ascending limb of the loop of Henle to inhibit active chloride reabsorption, thereby increasing sodium, chloride, and water excretion—it is FDA-approved for treating edema associated with congestive heart failure, cirrhosis, renal disease including nephrotic syndrome, and hypertension. 1

Mechanism of Action

• Furosemide is delivered to its site of action in the kidney tubules via active secretion through the nonspecific organic acid pump 2
• The drug acts at the luminal surface of the ascending limb of the loop of Henle by inhibiting the active reabsorption of chloride 2
• The diuretic response correlates with urinary drug concentration rather than plasma levels—meaning the drug must reach the tubular lumen to be effective 2

Chemical Properties and Formulation

• Furosemide is chemically classified as 4-chloro-N-furfuryl-5-sulfamoylanthranilic acid, an anthranilic acid derivative 1
• It is a white to off-white crystalline powder that is practically insoluble in water but freely soluble in dilute alkali solutions 1
• Available as oral tablets in 20 mg, 40 mg, and 80 mg strengths, and as intravenous formulation 1

FDA-Approved Indications

• Edema management: Treatment of fluid retention associated with congestive heart failure, cirrhosis of the liver, and renal disease including nephrotic syndrome—particularly useful when greater diuretic potential is needed 1
• Hypertension: May be used alone or in combination with other antihypertensive agents, though patients inadequately controlled with thiazides will likely not respond to furosemide alone 1

Pharmacokinetics

• Oral bioavailability is highly variable and often poor due to limited water solubility, potential site-specific absorption, and presystemic metabolism 2
• Peak diuretic effect occurs within 60-120 minutes after oral administration 3
• The drug is highly bound to plasma proteins (almost exclusively albumin), which restricts its volume of distribution 2
• Furosemide has two documented metabolites: furosemide glucuronide and saluamine 2

Clinical Effectiveness

• A dose as low as 20 mg produces significant diuretic and natriuretic effects in congestive heart failure patients, with peak effect within 60-120 minutes 3
• The drug increases sodium, potassium, and chloride excretion promptly in patients with chronic congestive heart failure 4
• Doses of 50 mg, 100 mg, and 200 mg orally produce progressively increasing diuretic responses 4
• Loop diuretics like furosemide can increase sodium excretion and urine output even when renal function is markedly impaired 5

Safety Profile

• In a large surveillance study of 2,367 hospitalized patients receiving furosemide, adverse reactions occurred in 10.1%, but only 14 cases (0.6%) were considered life-threatening 6
• Most common adverse reactions include intravascular volume depletion (4.6%), hypokalemia (3.6%), and other electrolyte disturbances (1.5%) 6
• The frequency of adverse reactions increases progressively with higher daily doses 6
• Coadministration of potassium supplements or potassium-sparing diuretics reduces the frequency and severity of hypokalemia 6
• Biochemical alterations may include elevated fasting blood sugar (particularly in diabetics), increased uric acid, and lowered plasma potassium—all reversible upon discontinuation 4

Important Clinical Considerations

• The response to furosemide is modulated by the individual's fluid and electrolyte balance 2
• Both acute and delayed tolerance can develop due to homeostatic mechanisms influencing fluid and electrolyte balances 2
• Long-term studies demonstrate that furosemide can maintain patients with chronic congestive heart failure without serious systemic toxicity when used at appropriate doses (40-160 mg daily) 4
• In chronic renal failure, furosemide is most helpful when impaired renal function coexists with nephrotic syndrome or chronic heart failure 5
• In acute renal failure, furosemide may convert oliguric to non-oliguric forms, facilitating patient management, though it does not affect mortality rates 5