Many of the interactions of furosemide are due to their effects on fluid and electrolyte balance. Diuretic induced hypokalaemia may enhance the toxicity of digitalis glycosides and increase the risk of arrhythmias with drugs that prolong the QT interval, such as astemizol, terfenadine, halofantrine, pimozide, and sotalol. Furosemide may also enhance the neuromuscular blocking action of competitive neuromuscular blockers, such as atracurium, probably by their hypokalaemic effect. The potassium-depleting effect of diuretics may be enhanced by corticosteroids, corticotrophin, beta2 agonists such as salbutamol, carbenoxolone, amphotericin B, or reboxetine.
Furosemide can enhance the effect of other antihypertensives, particularly the first-dose hypotension that occurs with alpha blockers or ACE inhibitors. Orthostatic hypotension associated with diuretics may be enhanced by alcohol, barbiturates or opioids. The antihypertensive effects of diuretics may be antagonized by drugs that cause fluid retention, such as corticosteroids, NSAIDs, or carbenoxolone; the neprotoxicity of NSAIDs may also be enhanced.
Furosemide may enhance the nephrotoxicity of cephalosporin antibacterials such as cefalotin and can enhance the ototoxicity of aminoglycoside antibacterials and other ototoxic drugs.
Furosemide should not usually be used with lithium since the association may lead to toxic blood concentrations of lithium. Other drugs for which increased toxicity has been reported when given with furosemide include allopurinol and tetracyclines. Furosemide may alter the requirements for hypoglycaemics in diabetic patients.
There has been report of symptomatic hyponatraemia associated with the use of hydrochlorothiazide or furosemide and carbamazepine.