Furosemide (Lasix): Loop Diuretic Patient Guide

What Furosemide Treats

Electrolytes to Monitor

How furosemide works – and why it takes salt with it

Furosemide acts on the thick ascending limb of the loop of Henle in the kidney, blocking the sodium-potassium-chloride (Na⁺-K⁺-2Cl⁻) co-transporter. This prevents salt and water from being reabsorbed back into the bloodstream, so they pass into the urine instead. The result is a large, fast-onset diuresis that typically starts within 30–60 minutes of an oral dose and peaks at 1–2 hours.

The same mechanism that removes sodium also causes the kidney to lose potassium, magnesium, and – to a lesser extent – calcium. This is why doctors monitor these electrolytes closely and why potassium or magnesium supplements are often prescribed alongside furosemide in long-term use.

When to hold furosemide – AKI and volume depletion

Furosemide can worsen kidney function if the blood volume falls too low, a condition called pre-renal acute kidney injury (AKI). Warning signs include: creatinine rising by more than 26 µmol/L (0.3 mg/dL) in 48 hours, urea rising disproportionately to creatinine (suggesting dehydration rather than intrinsic kidney disease), or the patient becoming dizzy on standing (postural hypotension).

As a general rule, furosemide should be temporarily withheld during episodes of vomiting, diarrhoea, or extreme heat – when additional fluid loss increases AKI risk. This is part of "sick day guidance" (sometimes called sick day rules): a set of instructions your doctor should provide when prescribing furosemide long-term. The medications most commonly listed for sick day suspension are furosemide, ACE inhibitors/ARBs, and metformin – sometimes called the "triple whammy" – because their combination sharply increases AKI risk during illness.

Current NICE guidance suggests stopping furosemide temporarily if creatinine rises above 30% from baseline, or if eGFR falls below 15 mL/min/1.73m², pending medical review.

Furosemide versus other diuretics

There are three main diuretic classes used in heart failure and fluid overload:

• Loop diuretics (furosemide, bumetanide): Strongest effect; used for acute and chronic fluid overload. Furosemide 40 mg orally ≈ bumetanide 1 mg in effect.
• Thiazide diuretics (bendroflumethiazide, indapamide): Moderate effect; mainly used for blood pressure rather than fluid removal. Lose effectiveness when eGFR falls below 30.
• Potassium-sparing diuretics (spironolactone, eplerenone): Weak diuresis alone, but combined with furosemide they reduce potassium loss and – in heart failure – have proven mortality benefit.

In resistant oedema, a combination of furosemide and spironolactone is often more effective than increasing the furosemide dose alone.

References

Sources cited on this page. PubMed links open the original abstract.

1. Jessup M, Brozena S. Heart failure. N Engl J Med. 2003;348(20):2007–2018. PMID 12748317 · doi:10.1056/NEJMra021498