What is the equivalent dose of furosemide (Lasix) when converting from spironolactone?

Spironolactone to Furosemide Conversion

There is no direct dose equivalency conversion between spironolactone and furosemide because they are fundamentally different diuretics with distinct mechanisms of action—spironolactone is an aldosterone antagonist acting on the distal tubule, while furosemide is a loop diuretic acting on the loop of Henle. 1

Why Direct Conversion Is Not Applicable

• Spironolactone and furosemide target different nephron segments and have complementary rather than interchangeable effects 1
• Spironolactone blocks aldosterone-mediated sodium reabsorption in the distal tubule and collecting duct, while furosemide inhibits the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle 1
• The two drugs are designed to be used together, not substituted for one another 1

Standard Combination Dosing Ratios (Not Conversion)

When these medications are used together—which is the recommended approach for conditions like cirrhotic ascites and heart failure—the established ratio is:

• 100 mg spironolactone : 40 mg furosemide given as a single morning dose 1, 2
• This ratio can be increased simultaneously (maintaining the 100:40 proportion) every 3-5 days if inadequate response, up to maximum doses of 400 mg/day spironolactone and 160 mg/day furosemide 1, 2
• This 100:40 ratio maintains adequate serum potassium levels and provides optimal diuresis 2

Clinical Context: When to Use Each Drug

For Cirrhotic Ascites

• Aldosterone antagonists (spironolactone) are the mainstay of therapy—loop diuretics alone as monotherapy are not recommended 1, 2
• Initial therapy should be spironolactone 100 mg plus furosemide 40 mg as a single morning dose 1, 2
• Combination therapy is superior to spironolactone monotherapy for recurrent ascites, providing faster control with lower hyperkalemia risk 2

For Heart Failure

• Diuretics (primarily loop diuretics like furosemide) are recommended for symptom relief in patients with fluid retention 1
• Initial furosemide dose is 20-40 mg IV for new-onset acute heart failure or those not on chronic diuretics 1
• Spironolactone (as an aldosterone antagonist) is added for its mortality benefit in heart failure with reduced ejection fraction, not primarily for diuresis 1

Critical Monitoring When Using Either Drug

• Check potassium and creatinine at 3 days, 1 week, then monthly for first 3 months 2
• Monitor for hyperkalemia risk factors: baseline creatinine >1.6 mg/dL, baseline potassium >5.0 mEq/L, concomitant ACE inhibitors, diabetes, elderly patients, dehydration 2
• Target weight loss should not exceed 0.5 kg/day without peripheral edema or 1 kg/day with edema 1, 2
• Stop diuretics if severe hyponatremia (<120-125 mmol/L), acute kidney injury, progressive renal failure, or marked hypotension develops 1

Common Pitfall to Avoid

Do not attempt to "switch" from spironolactone to furosemide or vice versa as if they were equivalent medications. 1 If a patient is on spironolactone alone and requires more diuresis, the correct approach is to add furosemide (starting at 40 mg/day) while continuing spironolactone, not to replace one with the other 1, 2