Furosemide is primarily used for the treatment of edema, but also in some cases of hypertension (where there is also kidney or heart impairment). It is often viewed as a first-line agent in most people with edema caused by congestive heart failure. Compared with furosemide, however, torsemide is associated with a lower risk of rehospitalization for heart failure and an improvement in New York Heart Association class of heart failure but no difference in the risk of death. Torsemide may also be safer than furosemide.
It is mainly excreted by tubular secretion in the kidney. In kidney impairment, clearance is reduced, increasing the risk of adverse effects. Lower initial doses are recommended in older patients (to minimize side-effects) and high doses may be needed in kidney failure. It can also cause kidney damage; this is mainly by loss of excessive fluid (i.e. dehydration), and is usually reversible.
Furosemide acts within 1 hour of oral administration (after IV injection, the peak effect is within 30 minutes). Diuresis is usually complete within 6–8 hours of oral administration, but there is significant variation between individuals.
The tendency, as for all loop diuretics, to cause low serum potassium concentration (hypokalemia) has given rise to combination products, either with potassium or with the potassium-sparing diureticamiloride (Co-amilofruse). Other electrolyte abnormalities that can result from furosemide use include hyponatremia, hypochloremia, hypomagnesemia, and hypocalcemia.
In the treatment of heart failure, many studies have shown that the long-term use of furosemide can cause varying degrees of thiamine deficiency, so thiamine supplementation is also suggested.
Although disputed, it is considered ototoxic: "usually with large intravenous doses and rapid administration and in renal impairment".
Other precautions include: nephrotoxicity, sulfonamide (sulfa) allergy, and increases free thyroid hormone effects with large doses.
Furosemide has potential interactions with these medications:
The action on the distal tubules is independent of any inhibitory effect on carbonic anhydrase or aldosterone; it also abolishes the corticomedullary osmotic gradient and blocks negative, as well as positive, free water clearance. Because of the large NaCl absorptive capacity of the loop of Henle, diuresis is not limited by development of acidosis, as it is with the carbonic anhydrase inhibitors.
Additionally, furosemide is a noncompetitive subtype-specific blocker of GABA-A receptors. Furosemide has been reported to reversibly antagonize GABA-evoked currents of α6β2γ2 receptors at μM concentrations, but not α1β2γ2 receptors. During development, the α6β2γ2 receptor increases in expression in cerebellar granule neurons, corresponding to increased sensitivity to furosemide.
Molecular weight (daltons) 330.7
% Bioavailability 47-70%
Bioavailability with end-stage renal disease 43 - 46%
The diuretic effects are put to use most commonly in horses to prevent bleeding during a race. Sometime in the early 1970s, furosemide's ability to prevent, or at least greatly reduce, the incidence of bleeding (exercise-induced pulmonary hemorrhage) by horses during races was discovered accidentally. In the United States of America, pursuant to the racing rules of most states, horses that bleed from the nostrils three times are permanently barred from racing. Clinical trials followed, and by decade's end, racing commissions in some states in the USA began legalizing its use on race horses. On 1 September 1995, New York became the last state in the United States to approve such use, after years of refusing to consider doing so. Some states allow its use for all racehorses; some allow it only for confirmed "bleeders". Its use for this purpose is still prohibited in many other countries.
Furosemide is also used in horses for pulmonary edema, congestive heart failure (in combination with other drugs), and allergic reactions. Although it increases circulation to the kidneys, it does not help kidney function, and is not recommended for kidney disease.
It is also used to treat congestive heart failure (pulmonary edema, pleural effusion, and/or ascites) in cats and dogs. It can also be used in an attempt to promote urine production in anuric or oliguric acute kidney failure.
Furosemide is injected either intramuscularly or intravenously, usually 0.5-1.0 mg/kg twice/day, although less before a horse is raced. As with many diuretics, it can cause dehydration and electrolyte imbalance, including loss of potassium, calcium, sodium, and magnesium. Excessive use of furosemide will most likely lead to a metabolic alkalosis due to hypochloremia and hypokalemia. The drug should, therefore, not be used in horses that are dehydrated or experiencing kidney failure. It should be used with caution in horses with liver problems or electrolyte abnormalities. Overdose may lead to dehydration, change in drinking patterns and urination, seizures, gastrointestinal problems, kidney damage, lethargy, collapse, and coma.
Furosemide should be used with caution when combined with corticosteroids (as this increases the risk of electrolyte imbalance), aminoglycoside antibiotics (increases risk of kidney or ear damage), and trimethoprim sulfa (causes decreased platelet count). It may also cause interactions with anesthetics, so its use should be related to the veterinarian if the animal is going into surgery, and it decreases the kidneys' ability to excrete aspirin, so dosages will need to be adjusted if combined with that drug.
Furosemide may increase the risk of digoxin toxicity due to hypokalemia.
The drug is best not used during pregnancy or in a lactating mare, as it has been shown to be passed through the placenta and milk in studies with other species. It should not be used in horses with pituitary pars intermedia dysfunction (Cushings).
Furosemide is detectable in urine 36–72 hours following injection. Its use is restricted by most equestrian organizations.
In April 2019, it was announced that Lasix would be banned from use within 24 hours of a horse racing starting in 2021.
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