Furosemide (INN) or frusemide (former BAN) is a loop diuretic used in the treatment of congestive heart failure and edema. It is most commonly marketed by Sanofi-Aventis under the brand name Lasix. It has also been used to prevent Thoroughbred and Standardbred race horses from bleeding through the nose during races.

Along with some other diuretics, furosemide is also included on the World Anti-Doping Agency's banned drug list due to its alleged use as a masking agent for other drugs.

Furosemide is primarily used for the treatment of two conditions: hypertension and edema. It is the first line agent in most people with edema due to congestive heart failure.

It is also sometimes used in the management of severe hypercalcemia in combination with adequate rehydration.

Although disputed, it is considered ototoxic: "usually with large parenteral doses and rapid administration and in renal impairment" Furosemide also can lead to gout due to hyperuricemia. Hyperglycemia is also a common side effect.