Lasix (furosemide) is a medication routinely used to prevent lung bleeding in horses with exercise-induced pulmonary hemorrhage (EIPH).

This bleeding disorder is prevalent in racehorses following bouts of high-intensity exercise. Veterinarians often recommend treating with Lasix on race days to minimize the risk of bleeding, with a reported 74.2% of racehorses in North America using the drug. [28]

Horses on Lasix have faster race times and earn more money over their career than horses not on the drug. [28] However, controversy surrounds the widespread use of Lasix in the racing industry.

While research shows that furosemide is an effective treatment for “bleeders,” some argue it should be banned as a performance-enhancing agent. This drug is also potent diuretic with a risk of side effects if it is not used properly.

This article will review how Lasix works for bleeders and post-race recovery in horses, the side effects of furosemide, and alternative options for managing horses with EIPH.

Exercise-Induced Pulmonary Hemorrhage

Horses that develop nosebleeds (epistaxis) after intense exercise may have exercise-induced pulmonary hemorrhage. Horse racing trainers have identified these horses as “bleeders” for centuries.

Modern fibre-optic endoscopes eventually allowed veterinarians to visualize the source of internal bleeding in the horse’s windpipe. [1] Today, practitioners recognize EIPH as a significant welfare concern for equine athletes.


EIPH is a hemorrhage of the pulmonary capillaries in the horse’s lung, resulting in blood in the alveoli and airways. This condition occurs during strenuous exercise and is common in event horses, barrel horses, and racehorses. [2]

Research suggests that nearly all racehorses experience EIPH at some point in their career. Studies that examine the airways of racehorses report blood in the trachea of 65% – 95% of subjects. [3]

However, only 5% of bleeders show outward signs of blood loss. [2] For this reason, exercise-induced pulmonary hemorrhage is likely underdiagnosed among racehorses.

EIPH Scale

Veterinarians grade the severity of EIPH cases on a scale of 1-4. [4]

Grade 0: No visible blood

Grade 1: Small dots or short streams of blood visible in the trachea

Grade 2: Multiple short streams of blood or one long stream of blood visible

Grade 3: Several streams of blood cover more than one-third of the trachea

Grade 4: Multiple streams of blood cover over 90% of the trachea, epistaxis

Why do Horses Bleed During Exercise?

Oxygen moves from the air in the horse’s lungs into their blood through pulmonary capillaries in the alveoli. The fragile walls of these blood vessels are only one cell thick to allow gas exchange. [5]

Strenuous exercise rapidly depletes oxygen in the horse’s muscles and organs. Heart rate increases to circulate more oxygenated blood, increasing blood pressure in the capillaries. [6]

The horse must also breathe more air to meet oxygen demands, increasing tidal volume. The high tidal volume and increased cardiac output can cause microscopic tears in pulmonary capillaries, breaching the blood-gas barrier. [5]

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How EIPH Impacts Horse Welfare

The horse’s body can absorb minor blood loss in pulmonary spaces without adversely affecting tissue oxygenation. While most performance horses experience some degree of EIPH, the majority will n