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Effects of Furosemide on Performance and Mineral Excretion in RacehorsesBy Kentucky Equine Research Staff · November 7, 2014

Exercised-induced pulmonary hemorrhage (EIPH), more commonly known as “bleeding,” is a malady that affects some racehorses. During the intense effort of training or racing, capillaries in the horse’s respiratory passages break and spill blood that can interfere with breathing. This blood can sometimes be seen dripping from the horse’s nostrils during and after a race. Blood is rarely extreme, but it presents a negative image to the public, and the condition is linked to horses running slower.  

To prevent EIPH and get the best level of performance, some trainers have their horses treated with furosemide (Lasix) before racing. With its diuretic action, Lasix increases urination and some horses apparently lost enough weight to have a positive effect on their racing speed.  

Kentucky Equine Research conducted several studies to determine the effects of chronic furosemide use. In one study designed to look at energetic efficiency, six Thoroughbred horses were used in a trial in which they were weighed before and after exercising on a high-speed treadmill. Some of the horses were given Lasix four hours before exercising, while others were used as an untreated control group. Weight loss was higher in treated horses. Heart rate, maximum oxygen consumption, and lactate production were higher during exercise in untreated than in treated horses, showing the untreated horses had to work harder and showed more signs of muscle fatigue. Treated horses were thought to perform more efficiently because of a loss of body weight.

A second study examined the effect of furosemide on urinary and fecal excretion of minerals in the 72 hours after administration. Fecal excretion of minerals was unchanged in treated horses. Though urinary excretion of magnesium and potassium was not affected, there were significant increases in urinary excretion of sodium, chloride, calcium, and phosphorus in treated horses. It is not known whether continued use of furosemide could affect bone strength and skeletal development in young horses through a disrupted calcium and phosphorus balance. Horses that are given furosemide on a regular basis might benefit from calcium supplementation, but more studies will be needed to confirm this.