Furosemide is a loop diuretic medication used to treat fluid build-up due to heart failure, liver scarring, or kidney disease. It had many trade names including Uremide, Lasix, Discoid, and Frusemide. Furosemide may also be used for the treatment of high blood pressure. It can be taken by injection into a vein or by mouth. When given intravenously, furosemide typically begins working within five minutes; when taken by mouth, it typically begins working within an hour.

Common side effects include feeling lightheaded while standing, ringing in the ears, and sensitivity to light. Potentially serious side effects include electrolyte abnormalities, low blood pressure, and hearing loss. Blood tests are recommended regularly for those on treatment. Furosemide is a type of loop diuretic that 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 2020, it was the nineteenth most commonly prescribed medication in the United States, with more than 26 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.