Furosemide

Furosemide is a loop diuretic and the most potent orally available medication for removing excess fluid. It is used for edema (fluid retention) of various causes, particularly in heart failure, kidney disease, and liver cirrhosis. It can also be used for high blood pressure, although thiazide diuretics are generally preferred here.

The name is derived from its site of action: the loop of Henle in the kidney. Furosemide causes rapid and powerful excretion of water. It is available as tablets, solution for injection, and drops. Well-known products include Lasix®, Furosemid-ratiopharm®, and Furosemid AL®.

Furosemide has a rapid onset of action and is therefore also used in emergency situations (e.g., acute pulmonary edema).

Furosemide inhibits the sodium-potassium-2-chloride cotransporter (NKCC2) in the loop of Henle:

Mechanism of action:

• Inhibits the reabsorption of sodium, potassium, and chloride in the loop of Henle
• Large amounts of water follow the electrolytes osmotically
• Powerful diuresis (water excretion)
• Rapid reduction of blood volume

Potency:

• Most potent oral diuretic
• Can excrete up to 15-25% of filtered sodium
• For comparison: thiazides only 5-8%

Additional effects:

• Venous vasodilation (preload reduction) – acts even before diuresis
• Increased calcium and magnesium excretion
• Increased uric acid retention

Onset of action: Oral: 30-60 minutes. Intravenous: 5-15 minutes. Duration of action: 4-6 hours. Afterward, counter-regulation (rebound sodium retention) may occur.

Furosemide is suitable for:

• Edema in heart failure
• Edema in kidney disease
• Edema in liver cirrhosis
• Pulmonary edema (emergency)
• High blood pressure (when thiazides are not sufficient)
• Forced diuresis in poisoning

Particularly suitable for:

• Heart failure with fluid overload
• Renal insufficiency (GFR below 30 ml/min – thiazides have hardly any effect here)
• Acute pulmonary edema (emergency)
• Severe edema that does not respond to thiazides

Not suitable:

• Sole long-term therapy for high blood pressure (thiazides preferred)
• Anuria (absent urine production)
• Severe hypokalemia
• Severe hyponatremia
• Precomatose hepatic encephalopathy

Edema:

• Starting dose: 20-40 mg once daily (in the morning)
• Usual dose: 40-80 mg daily
• Maximum dose: 600 mg daily (in severe renal insufficiency)
• Can be divided into 2 doses if needed

High blood pressure:

• 20-40 mg once or twice daily

Pulmonary edema (emergency):

• 40-80 mg i.v.

Available strengths:

• 20 mg, 40 mg, 250 mg, 500 mg tablets
• Solution for injection
• Oral solution

Dose adjustment in renal insufficiency: With a GFR below 30 ml/min, higher doses are often necessary (up to 250-500 mg).

How to take:

• Take in the morning (to avoid needing to urinate at night)
• If taken twice daily: in the morning and at midday (not in the evening)
• With or without food
• With sufficient water

Important notes:

• Regular laboratory checks: electrolytes (potassium!), kidney function, blood sugar
• Drink enough fluids (as instructed by your doctor)
• Potassium-rich diet (bananas, potatoes, nuts)
• Check your weight regularly (indicates fluid loss/fluid overload)
• In case of severe diarrhea or vomiting: contact your doctor (risk of dehydration)
• Use sun protection (photosensitivity possible)

Furosemide must not be taken in case of:

• Anuria (no urine production)
• Severe hypovolemia or dehydration
• Severe hypokalemia
• Severe hyponatremia
• Precomatose or comatose hepatic encephalopathy
• Hypersensitivity to furosemide or sulfonamides

Caution with:

• Diabetes mellitus (increase in blood sugar possible)
• Gout/hyperuricemia (increase in uric acid)
• Liver cirrhosis with ascites (close monitoring)
• Concomitant use of ototoxic medications
• Prostatic hyperplasia (urinary retention possible)
• Porphyria

Pregnancy: Only if strictly indicated. Furosemide crosses the placenta and can increase diuresis in the fetus.

Common (1-10%):

• Hypokalemia (potassium deficiency) – most important side effect!
• Hyponatremia
• Dehydration
• Hypotension
• Elevated uric acid
• Metabolic changes (blood sugar, blood lipids)

Uncommon (0.1-1%):

• Dizziness, headache
• Muscle cramps
• Gastrointestinal complaints
• Skin rash
• Photosensitivity
• Hearing disorders (especially with rapid i.v. administration)

Rare:

• Severe electrolyte disturbances
• Pancreatitis
• Changes in blood count
• Worsening of kidney function
• Ototoxicity (irreversible at very high doses)

Important: Potassium loss is a common side effect and can become dangerous. Regular potassium checks are mandatory! If necessary, potassium supplementation or combination with a potassium-sparing diuretic.

Dangerous combinations:

• Aminoglycoside antibiotics (gentamicin) → increased ototoxicity and nephrotoxicity
• Cisplatin → increased kidney and hearing damage
• Lithium → increased lithium levels (toxic!)

Caution with:

• Digitalis preparations → potassium loss increases digitalis toxicity!
• ACE inhibitors/sartans → increased hypotension, kidney dysfunction
• NSAIDs → reduced diuretic effect, kidney damage
• Corticosteroids → increased potassium loss
• Antidiabetic drugs → reduced blood sugar-lowering effect
• Sucralfate → reduced furosemide absorption (keep 2 hours apart)

Important: When digitalis (digoxin) is taken at the same time, potassium levels must be monitored particularly carefully. Hypokalemia increases the risk of life-threatening cardiac arrhythmias.