Furosemide

Is furosemide safe?

Furosemide is safe when the dose fits the person and blood tests keep track of it. It is the drug that takes fluid off the lungs and legs in heart failure, and for many patients it is the difference between breathing and not. In 2024 it was the 29th most prescribed medicine in the United States, with about 20.1 million prescriptions (ClinCalc).

The danger is not a single overdose, which rarely kills, but doing its job too well. The US label for Lasix opens with a boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary: in excessive amounts it can cause "profound diuresis with water and electrolyte depletion." We score it 70, well below the gentler water pill hydrochlorothiazide (83). A lower score does not mean you should stop taking it; for heart failure it is often essential.

Why furosemide needs blood tests

Furosemide blocks salt reabsorption in the loop of Henle, the part of the kidney that handles the most salt. That is why it is so much stronger than thiazides, and why the label asks for potassium, sodium, kidney function and bicarbonate to be checked "frequently during the first few months" and regularly after that.

What goes wrong, and the signs to watch for:

• Low potassium: muscle cramps, weakness, palpitations. More likely with high doses, liver cirrhosis, steroids, laxatives or large amounts of liquorice. In people on digoxin it raises the risk of rhythm problems.
• Low sodium and dehydration: thirst, dry mouth, confusion, dizziness, peeing less. The NHS lists dark urine and sunken eyes as signs of serious dehydration.
• Kidney strain: the label says to stop if kidney function keeps worsening in severe kidney disease.
• Gout and blood sugar: uric acid rises, and blood sugar can go up in diabetes.

Many patients take it with a potassium-sparing drug such as spironolactone, which partly offsets the potassium loss.

Falls and older people

Older people lose fluid faster than they notice, and getting up at night to pee is when many falls happen. In a study of 1,785 Boston nursing home residents, the odds of falling the day after a loop diuretic was started or increased were about 2.5 times higher, or one extra fall for every 271 changes. The label adds that excessive fluid loss can cause circulatory collapse and clots, particularly in elderly patients, and that men with an enlarged prostate can go into acute urinary retention.

Practical steps: take it in the morning, get up slowly, and weigh yourself daily if your heart team asks you to. A fall of more than a kilo or two in a few days, with dizziness, can mean too much fluid is coming off.

Interactions that matter

The label lists many, and several are serious:

The NSAID risk is well measured. A UK study of 487,372 people on blood pressure drugs found that taking a diuretic plus an ACE inhibitor or ARB plus an NSAID raised the rate of acute kidney injury by 31%, and by 82% in the first month. Over-the-counterOver-the-counterA medicine you can buy without a prescription, in a pharmacy or shop, with directions on the pack for safe use without a doctor.More in the glossary cold remedies and appetite suppressants are also on the label's list to avoid in people with high blood pressure.

Misuse and recalls

Furosemide is not addictive, but it is misused. Athletes take diuretics to make weight and to dilute urine samples, and WADA bans them in and out of competition. People with eating disorders take them to lose weight, which is only water and can cause dangerous salt imbalances.

In March 2026 one US generic maker, Leading Pharma, recalled 9,384 bottles of 80 mg tablets because of an N-nitroso-furosemide impurity above the accepted limit. This was a manufacturing problem with one product, not a problem with the drug itself. If you are worried about your tablets, ask your pharmacist rather than stopping.

Who should be careful

• People who cannot pass urine at all (anuria), in whom it is contraindicatedContraindicationA situation in which a medicine should not be used, because the risk clearly outweighs any benefit, such as a known allergy to it.More in the glossary.
• People with liver cirrhosis, in whom the label says treatment is best started in hospital.
• Anyone with a sulfa drug allergy, lupus, gout or diabetes.
• Pregnant women: the NHS says it is not usually used in pregnancy.
• People on lithium, digoxin, an ACE inhibitor or ARB, or regular NSAIDs.

Get urgent help for fainting, a very fast or irregular heartbeat, severe muscle weakness, sudden hearing loss, or a blistering rash.

How we scored it

Track record (95) and dependence (90) are strong, and overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary (78) is fair because acute overdoses rarely kill. Serious side effects (62), interactions (55) and regulatory warnings (48) pull the score down: the boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary reflects real, common harms from too much fluid and salt loss, and the list of risky drug combinations is long.