Can Lasix Cause Low Sodium?
Yes, Lasix (furosemide) can cause hyponatremia (low sodium), though this occurs less frequently and less severely than with thiazide diuretics. 1
Mechanism and Risk Profile
Furosemide causes hyponatremia through electrolyte depletion during diuretic therapy, particularly in patients receiving higher doses with restricted salt intake. 1 The FDA label explicitly lists hyponatremia as a sign of fluid or electrolyte imbalance that requires monitoring in all patients receiving furosemide. 1
However, the risk is substantially lower compared to thiazide diuretics:
- In a comprehensive review of 129 cases of severe diuretic-induced hyponatremia (sodium <115 mEq/L), thiazides were responsible for 94% of cases, while furosemide accounted for very few. 2
- Thiazide-induced hyponatremia typically develops within 14 days, whereas none of the furosemide-treated patients developed hyponatremia in this timeframe. 2
- In a direct comparison study, an elderly woman who developed severe hyponatremia on thiazides was safely rechallenged with furosemide without recurrent hyponatremia. 3
Clinical Context Where Hyponatremia Risk Increases
Cirrhosis with ascites: Hyponatremia occurs in 8-30% of patients treated with diuretics for ascites, related to impaired free water excretion. 4 When hydrochlorothiazide is added to the combination of spironolactone and furosemide, it can cause rapid development of hyponatremia. 4
Advanced heart failure: In patients with acute decompensated heart failure and dilutional hyponatremia (sodium ≤135 mEq/L), furosemide administration is associated with variable natriuretic responses. 5 However, in the EFFUSE-FLUID trial of patients with syndrome of inappropriate antidiuresis (SIAD), furosemide combined with fluid restriction did not worsen hyponatremia compared to fluid restriction alone. 6
Monitoring Requirements
The FDA mandates frequent monitoring for signs of hyponatremia: 1
- Check serum electrolytes (particularly sodium) frequently during the first few months of therapy and periodically thereafter
- Monitor for clinical signs: dryness of mouth, thirst, weakness, lethargy, drowsiness, restlessness, muscle pains or cramps, hypotension, oliguria, tachycardia, or gastrointestinal disturbances
- Serum and urine electrolyte determinations are particularly important when patients are vomiting profusely or receiving parenteral fluids
Key Clinical Pitfalls
Hypokalemia is actually more common than hyponatremia with furosemide. 7, 1 The FDA label emphasizes that hypokalemia may develop especially with brisk diuresis, inadequate oral electrolyte intake, cirrhosis, or concomitant use of corticosteroids. 1
Combination therapy with potassium-sparing diuretics (like spironolactone) helps maintain electrolyte balance and is the recommended approach in cirrhosis, typically using a 100 mg spironolactone to 40 mg furosemide ratio. 4, 7
In overdose situations, the principal signs include dehydration, blood volume reduction, hypotension, and electrolyte imbalance including hyponatremia, which are extensions of furosemide's diuretic action. 1