Furosemide, sold under the brand name Lasix among others, is a loop diuretic medication used to treat edema due to heart failure, liver scarring, or kidney disease. Furosemide may also be used for the treatment of high blood pressure. It can be taken intravenously or orally. When given intravenously, furosemide typically takes effect within five minutes; when taken orally, it typically metabolizes within an hour.

Common side effects include orthostatic hypotension (decrease in blood pressure while standing, and associated lightheadedness), tinnitus (ringing in the ears), and photosensitivity (sensitivity to light). Potentially serious side effects include electrolyte abnormalities, low blood pressure, and hearing loss. It is recommended that serum electrolytes (especially potassium), serum CO₂, creatinine, BUN levels, and liver and kidney functioning be monitored in patients taking furosemide. It is also recommended to be alert for the occurrence of any potential blood dyscrasias.

Furosemide works by decreasing the reabsorption of sodium by the kidneys. Common side effects of furosemide injection include hypokalemia (low potassium level), hypotension (low blood pressure), and dizziness.

Furosemide was patented in 1959 and approved for medical use in 1964. It is on the World Health Organization's List of Essential Medicines. In the United States, it is available as a generic medication. In 2023, it was the 29th most commonly prescribed medication in the United States, with more than 19 million prescriptions. In 2020/21 it was the twentieth most prescribed medication in England. It is on the World Anti-Doping Agency's banned drug list due to concerns that it may mask other drugs. It has also been used in race horses for the treatment and prevention of exercise-induced pulmonary hemorrhage.

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 because of its anti-vasoconstrictor and diuretic effects. Compared with furosemide, however, torasemide (aka "torsemide") has been demonstrated to show improvements to heart failure symptoms, possibly lowering the rates of rehospitalization associated with heart failure, with no difference in risk of death. Torsemide may also be safer than furosemide. Providing self-administered subcutaneous furosemide has been found to reduce hospital admissions in people with heart failure, resulting in significant savings in healthcare costs.

Furosemide is also used for liver cirrhosis, kidney impairment, nephrotic syndrome, in adjunct therapy for swelling of the brain or lungs where rapid diuresis is required (IV injection), and in the management of severe hypercalcemia in combination with adequate rehydration.

In chronic kidney diseases with hypoalbuminemia, furosemide is used along with albumin to increase diuresis. It is also used along with albumin in nephrotic syndrome to reduce edema.

Furosemide 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.^{[citation needed]}