Lasix (Furosemide) 20 mg / 40 mg / 80 mg Tablets: Prescription Loop Diuretic for Acute and Chronic Fluid Overload, Heart Failure, and Hypertension — Rapid Onset

Information last reviewed: May 2026 — for educational purposes only.

What Is Lasix (Furosemide)?

Furosemide, sold under the brand name Lasix, is a loop diuretic — the most potent class of diuretic medications. It inhibits the Na-K-2Cl co-transporter in the thick ascending limb of the loop of Henle in the kidney, preventing reabsorption of sodium, chloride, and water. This produces a rapid and robust diuresis, making furosemide the drug of choice for acute pulmonary oedema, congestive heart failure, and states of severe fluid retention unresponsive to thiazide diuretics.

Prescription (Rx) Status

Furosemide is a prescription-only medicine. The oral tablet form is widely available as a cheap generic. IV formulations are used in hospital settings for acute decompensated heart failure and are administered under medical supervision. Furosemide is potent and can cause rapid electrolyte depletion, dehydration, and acute kidney injury if misused.

Uses of Furosemide

Furosemide is used for: congestive heart failure (CHF) with fluid congestion; oedema from renal, hepatic, or cardiac disease; hypertension (less common — usually when other agents fail); acute pulmonary oedema (emergency IV use); and hypercalcaemia. It causes significant urinary losses of potassium, sodium, magnesium, and calcium, requiring electrolyte monitoring.

Tablet Strengths

Furosemide oral tablets are available in: 20 mg, 40 mg, and 80 mg. Oral solution (10 mg/mL) and injectable solutions (10 mg/mL) are also available. Typical starting oral dose: 20–40 mg once or twice daily. Maximum daily oral dose: up to 600 mg in severe oedema (specialist use).

Price

Generic furosemide is extremely affordable. A 30-day supply of 40 mg tablets typically costs $4–$12 at major pharmacy chains. IV furosemide costs vary by hospital setting.

Frequently Asked Questions

Q: How quickly does Lasix work?
A: Oral furosemide typically begins working within 30–60 minutes, with peak diuretic effect at 1–2 hours and duration of 6–8 hours. IV furosemide acts within minutes.

Q: Can furosemide deplete potassium dangerously?
A: Yes — hypokalaemia is a significant risk. Electrolytes should be monitored regularly, and potassium supplementation or a potassium-sparing diuretic may be co-prescribed.

Q: Is furosemide the same as Lasix?
A: Yes. Lasix is the original brand name; furosemide is the generic name. Both are the same drug.

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