What is furosemide?
Furosemide is a loop diuretic and the most powerful water-shifting agent in the range. It blocks the sodium-potassium-chloride transporter in the ascending limb of the loop of Henle in the kidney. Because a very large share of filtered sodium is recovered there, the effect is correspondingly massive: sodium stays in the urine and water follows osmotically.
The effect is fast, strong and precisely for that reason more dangerous than it looks. What gets flushed out here is not only water.
This information is for research and educational purposes only. No medical advice.
Comparison
Fluid removal at different sites
Furosemide and torsemide act in the loop of Henle, spironolactone at the mineralocorticoid receptor. Tap a card to open its entry.
Simplified from the targets in ChEMBL. Furosemide and torsemide inhibit the same transporter, a head-to-head trial showed no difference in mortality. 82
How it works
The blocked transporter normally pulls sodium, potassium and chloride back out of the urine together. Switch it off and all three stay in the urine. On top of that, sodium flow further down the nephron rises, which drives exchange against potassium and amplifies the potassium loss. Magnesium and calcium go along too.
That is why a potassium drop is not a possible side effect but the predictable consequence of the mechanism. It shows up as muscle cramps, weakness and in the worse case as cardiac arrhythmia. People in the bodybuilding scene have died from exactly this. It is not a theoretical risk, it is the documented main reason this substance deserves respect.
Competition note: diuretics are on the WADA prohibited list year-round. That is not only because of the water loss but because they act as masking agents for other substances. Anyone competing in a tested federation should know that.
Where it acts
Where furosemide acted in studies
Tap a point or a card. Colour = NKCC2, grey = observed in studies.
Targets in the ChEMBL database8
Compound CHEMBL35: one recorded mechanism, Inhibitor, direct binding.
- Gene
- SLC12A1
- UniProt
- Q13621
- ChEMBL
- CHEMBL1874
- Action
- Inhibitor, direct binding
Use and dosing
Dosing
Dosing, titration and syringe
Loading the dosing information …
- Getting started and stepping up, drawn as a staircase
- Units on the U-100 syringe for every step
- How long one vial lasts
What the research shows
The evidence revolves almost entirely around heart failure. It is remarkably sober. DOSE (NEJM 2011) tested two questions at once: bolus or continuous infusion as well as low or high dose. No significant difference was found in the co-primary endpoints. The high dose removed fluid faster but came with larger shifts in kidney values.
TRANSFORM-HF (JAMA 2023) compared torsemide with furosemide in over 2,800 patients and found no difference in all-cause mortality. The long-held assumption that torsemide is superior was therefore not confirmed.
ADVOR (NEJM 2022) and CLOROTIC (European Heart Journal 2023) examined combinations: added acetazolamide clearly improved decongestion, an added thiazide produced more weight loss but worse kidney values. The message from both is the same: more diuresis is not automatically better, it has a price.
Study results
What the trials measured
Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.
Creatinine after 72 hours1
Mean rise in µmol/l, 308 patients with acute heart failure, given into a vein
None of the differences was significant, symptoms included. The high level removed more fluid and raised kidney values for a while.
Decongestion and course3
519 patients, loop diuretic plus acetazolamide or placebo
More decongestion, but no difference in death or readmission. Worse kidney values, low potassium and low blood pressure were about equally common in both groups.
Weight after 72 hours4
230 patients with acute heart failure, furosemide plus thiazide or placebo
More weight loss and urine, but no better breathlessness. Kidney values got worse much more often with the thiazide (see side effects).
Furosemide versus torsemide2
2,859 patients after a hospital stay for heart failure
No significant difference. The authors name dropouts, switching between groups and irregular use as limitations.
Research status
Where the research stands
Furosemide has been approved since 1966. On ClinicalTrials.gov 438 trials list furosemide among their interventions. Shown are the four registered trials from this entry, all on heart failure. As of 26 Oct 2026. 7
- Preclinical Target: the transporter NKCC2 in the loop of Henle
- Phase 1 no registered trial
- Phase 2 no registered trial
- Phase 3 3 trials, 3 completed
- Approval since 1966 according to ChEMBL
Show all 4 phase 3 and 4 trials on a timelineHide timeline
DOSE · NCT00577135 · 308 participants, started Feb 2008, primary endpoint reached Jan 2010.
Registry entry ↗ · Results: Felker et al., New England Journal of Medicine, 2011
CLOROTIC · NCT01647932 · 232 participants, started Oct 2014, primary endpoint reached Dec 2019. Tests several agents.
Registry entry ↗ · Results: Trullàs et al., European Heart Journal, 2023
TRANSFORM-HF · NCT03296813 · 2,973 participants, started Jul 2018, primary endpoint reached Jul 2022. Tests several agents.
Registry entry ↗ · Results: Mentz et al., JAMA, 2023
ADVOR · NCT03505788 · 519 participants, started Nov 2018, primary endpoint reached Jan 2022. Tests several agents.
Registry entry ↗ · Results: Mullens et al., New England Journal of Medicine, 2022
Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026.
Side effects
- Potassium drop: the documented main danger. Muscle cramps and weakness are the early signs, cardiac arrhythmia the serious consequence.
- Magnesium and sodium loss: go along with it and amplify the rhythm problems.
- Blood pressure drop and dizziness: mostly with too rapid fluid removal.
- Worsened kidney values: with overly aggressive use, because circulating volume gets too small.
- Raised uric acid: can trigger a gout attack.
- Ototoxicity: hearing disturbance at very high doses or rapid intravenous administration.
Tolerability
Kidney values in CLOROTIC
Out of 100 participants, in how many kidney values got worse on furosemide plus thiazide or placebo.
| Event | Added placebo | Added thiazide |
|---|---|---|
| Kidney function worsened4 | 17% | 47% |
Low potassium was about equally common in both groups in CLOROTIC and ADVOR. In DOSE the high level raised kidney values for a while.431
Storage
Store dry, at room temperature and protected from light. Keep out of reach of children.
