The most common undesirable effect is fluid and electrolyte imbalance. Other undesirable effects are relatively uncommon.
Blood and lymphatic system disorders:
Eosinophilia is rare. Occasionally, thrombocytopenia may occur. In rare cases, leucopenia and, in isolated cases, agranulocytosis, aplastic anaemia or haemolytic anaemia may develop.
Bone marrow depression has been reported as a rare complication and necessitates withdrawal of treatment.
Immune system disorders:
Severe anaphylactic or anaphylactoid reactions (e.g. with shock) occur rarely.
The incidence of allergic reactions, such as skin rashes, photosensitivity, vasculitis, fever, interstitial nephritis or shock is very low, but when these occur treatment should be withdrawn.
Metabolism and nutrition disorders:
Electrolyte and water balance may be disturbed as a result of diuresis. Furosemide causes increased excretion of sodium and chloride and consequently water, and hyponatraemia may occur. The diuretic action of furosemide may lead to or contribute towards hypovolaemia and dehydration, especially in elderly patients. Severe fluid depletion may lead to haemoconcentration with a tendency for thromboses to develop.
Excretion of other electrolytes is increased, and hypokalaemia, serum calcium depletion and hypomagnesaemia may occur. Symptomatic electrolyte disturbances and metabolic alkalosis may develop following gradual electrolyte depletion or acute severe electrolyte losses during higher dose therapy. Warning signs of electrolyte disturbances include increased thirst, headache, hypotension, confusion, muscle cramps, tetany, muscle weakness, disorders of cardiac rhythm and gastrointestinal symptoms. Pre-existing metabolic alkalosis (e.g. in decompensated cirrhosis of the liver) may be aggravated by furosemide treatment.
Serum cholesterol and triglyceride levels may rise during furosemide treatment. During long-term therapy they will usually return to normal within six months.
Furosemide may provoke hyperglycaemia and glycosuria but less so than thiazide diuretics. Glucose tolerance may decrease with furosemide. In patients with diabetes mellitus, this may lead to a deterioration of control; latent diabetes mellitus may become manifest.
Furosemide can increase serum uric acid levels and may precipitate attacks of gout in some patients.
Psychiatric/Nervous system disorders:
Rarely paraesthesia may occur. Symptoms of hypotension may include dizziness, light-headedness, sensation of pressure in the head, headache, drowsiness, concentration impairment and slowed reactions. Headache, lethargy or confusion may be warning signs of electrolyte disturbances.
Visual disturbances, blurred vision.
Ear and labyrinth disorders:
Hearing disorders, including deafness and tinnitus, may occur in rare cases, particularly in patients with renal failure, hypoproteinaemia (e.g. nephritic syndrome) and/ or when intravenous furosemide has been given too rapidly. Although symptoms are usually transient, permanent deafness may occur, especially in patients treated with other ototoxic medications.
Cardiac rhythm disturbances may occur as a consequence of electrolyte imbalance.
If furosemide is administered to premature infants during the first weeks of life, it may increase the risk of persistence of patent ductus arteriosus.
Hypotension and orthostatic hypotension may occur, especially in patients taking other medications which lower blood pressure.
Allergic vasculitis has been reported very rarely.
Nausea, vomiting, diarrhoea and dry mouth may occur but are not usually severe enough to necessitate withdrawal of treatment.
Hepatic encephalopathy in patients with hepatocellular insufficiency may occur.
In isolated cases, intrahepatic cholestasis, an increase in liver transaminases or acute pancreatitis may develop.
Skin and subcutaneous tissue disorders:
Skin and mucous membrane reactions may occasionally occur e.g. pruritis, urticaria, other rashes or bullous lesions, erythema multiforme, bullous pemphigoid, Stevens-Johnson syndrome, toxic epidermal necrolysis, exfoliative dermatitis, purpura, AGEP (acute generalized exanthematous pustulosis) and DRESS (Drug rash with eosinophilia and systemic symptoms).
Serum calcium levels may be reduced, muscle spasms or muscle weakness may indicate electrolyte disturbances. In very rare cases tetany has been observed.
Renal and urinary disorders:
Treatment with furosemide may lead to transient increases in blood creatinine and urea levels. Renal failure may occur as a consequence of fluid and electrolyte depletion, especially during concurrent treatment with NSAIDs or nephrotoxic medications.
Increased production of urine may provoke or aggravate complaints in patients with an obstruction of urinary outflow. Acute retention of urine with possible secondary complications may occur, for example, in patients with bladder emptying disorders, prostatic hyperplasia or narrowing of the urethra (see Precautions).
Nephrocalcinosis/nephrolithiasis has been reported in premature infants, and in adults, generally after long-term therapy.
There have been very rare reports of interstitial nephritis.
Asthenia, malaise, fever.
Following intramuscular injection, local reactions such as pain may occur.