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 not suffer any ill effects. [2]

A study of 744 Thoroughbreds in Australia found no association between grades 1-3 of EIPH and performance. However, horses with grade 4 EIPH had shortened racing careers. [7]

Some research suggests that EIPH is progressive. Although minor blood loss may not immediately impact equine health, repetitive trauma can produce scar tissue that leads to permanent lung damage. [2]

There is a possible link between EIPH and sudden deaths during racing. However, some researchers question whether these cases involved a different primary cause of death, such as cardiac arrest. [8]

Lasix for Horses

Lasix is a brand name for furosemide. This medication is a short-acting loop diuretic (water pill) administered orally or via injection to horses before exercise. [9]

Furosemide decreases fluid content in the horse’s body by increasing urine production. This reduces blood pressure and decreases pulmonary hemorrhage. [9]

Research supports furosemide as an effective treatment for reducing the severity of EIPH in horses. In over 60% of cases, furosemide treatment improves symptoms by at least one grade on the EPIH scale. [10]

Treating EIPH with Furosemide

Veterinarians administer furosemide 4 hours before planned exertion. The recommended dosage varies depending on the body weight and medical history of the horse.

Typical oral dosages are approximately 0.5 mg/lb body weight (1.0 mg/kg). This is equivalent to 500 mg for an 1100 lb (500 kg) horse.

After administration, trainers typically withhold water from the horse until after the event. [1]

Before recent changes to industry guidelines, racehorse trainers used Lasix preventatively in nearly 90% of all horses racing in the US. [1]

How Furosemide Works in Horses

Furosemide decreases sodium absorption in the kidneys, which causes the horse to excrete more urine. This medication also interferes with calcium and magnesium transport. [9]

Horses treated with Lasix lose 40 to 50 times more sodium and chloride in their urine within the first four hours following drug administration compared to the average horse. In addition, horses excrete more calcium in their urine for up to 72 hours after treatment. [9]

Excess water loss due to increased urination impacts body weight. In one study, horses treated with 150mg of furosemide lost 28 pounds in 4 hours. [11]

Water makes up 65-75% of your horse’s blood volume. When the horse excretes more water through urine, the reduction in overall blood volume decreases the horse’s blood pressure. [9]

Elevated blood pressure in the pulmonary capillaries is a direct cause of EIPH. Lowering blood pressure is an effective way to limit the severity of lung bleeding. [1]

Furosemide Side Effects

The extreme water loss caused by this diuretic can increase the risk of dehydration and other negative outcomes associated with poor hydration status. [12]

While most trainers offer free access to water after exercise, studies show that horses do not increase water intake for 20 hours. Horses treated with furosemide do not return to normal hydration levels for at least 24 hours. [13]

The 1.5 – 2.5% dehydration observed in some furosemide studies is enough to shift fluid balance in the horse’s body. Fluid loss in the intestinal tract is associated with an increased risk of colic. [14]

Some studies suggest that furosemide decreases calcium absorption, but existing research is inconclusive on whether long-term administration can impact bone health. [15]

Multiple studies show that furosemide decreases plasma concentrations of potassium and other electrolytes for at least 8 hours after administration. [16]

Potassium is involved in transmitting electrical signals to the muscles and heart. In rare circumstances, Lasix may contribute to abnormal heart rhythms. [17]

Does My Horse Need Lasix?

Consult with your veterinarian to determine whether furosemide is recommended for your horse. Your veterinarian can diagnose EIPH with an endoscopy or broncho-alveolar lavage (BAL). [2]

Your horse does not need to be a visible bleeder to be a candidate for Lasix. Other signs of EIPH include coughing after exercise and slower recovery.

Horses with EIPH typically take longer than 60 seconds to return to normal respiration rates following exercise. [2] Poor performance is another early indicator of EIPH.

Proper conditioning is essential for cardiovascular health. Reducing dust and allergens in your horse’s environment will also support respiratory function. [2]

Managing Horses on Furosemide

If your horse is being treated with Lasix, encourage them to drink water after the competition or event to prevent dehydration. Never give Lasix to a dehydrated horse. [18]

The mineral sodium plays a role in regulating thirst. Ensure your horse is getting enough salt in their diet to support hydration.

Using the diuretic Lasix also causes horses to excrete electrolyte minerals via urination. These minerals must be replaced with electrolyte supplementation to support electrolyte balance and to encourage water consumption. [16]

Mad Barn’s Performance XL: Electrolytes is a balanced electrolyte supplement scientifically designed to replace minerals lost through sweat, heat stress, or excess urination due to Lasix.

Performance XL: Electrolytes is a palatable supplement that provides sodium, potassium, calcium, and magnesium, as well as Vitamin E to support exercise recovery.

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Lasix Controversy in Horse Racing

Lasix has a long history of use as a race-day medication on nearly every track in North America. [1] Yet recent debate questions whether furosemide is appropriate or safe for racehorses. [2]

Recent investigations into the effects of furosemide on racehorses were triggered by a sudden spike in horse deaths at the Santa Anita racetrack. [19]

During the first three months of 2019, twenty-two horses died at Santa Anita. Nineteen of those horses used Lasix, and the remaining three didn’t have medication records because they hadn’t raced. [19]

Ultimately, an investigation could not establish a direct link between the deaths and furosemide. Other circumstances, such as unusually wet track conditions, may have contributed to the increased breakdowns at Santa Anita. [19]

However, one review found that Thoroughbred racehorses medicated with furosemide on race day had 62% greater odds of sudden death. [20]

Researchers proposed that furosemide may cause sudden death in horses due to fatal arrhythmia. Human research suggests that diuretic-induced electrolyte imbalances increase the risk of this condition. [21]

Further investigation is necessary to evaluate the health risks of furosemide in racehorses. [20]

Does Lasix Enhance Performance in Racehorses?

Controversy also surrounds Lasix as a potential performance-enhancing drug in the racing industry. [22]

Some stakeholders argue that the weight loss associated with this diuretic enhances performances and creates an unfair advantage on race day.

Lighter horses take longer to fatigue, and research shows that horses treated with furosemide expend less energy and have less lactic acid build-up after a race. [22]

Race records also show that horses on Lasix are more likely to win or finish in the top three of a race than horses not on Lsaix. [28] Lasix also improves 6-furlong race times by a difference roughly equal to 3 to 5.5 lengths. [28]

While horses prone to bleeding do perform better on Lasix, many veterinarians argue this medication is essential for their welfare. [1]

Lasix Bans in Horse Racing

Furosemide is not on the list of banned substances for performance horses competing in USEF-sanctioned events. The USEF Drugs & Medications Guidelines actually advocate using furosemide to encourage urination when collecting samples for drug testing.

However, Lasix bans exist in many countries, including the United Kingdom, Japan, and Germany. [23] And recent anti-doping legislation is leading the U.S. horse racing industry away from decades of widespread furosemide use. [24]

Two racing divisions in the United States have started to phase out the use of Lasix. Despite initial concerns, Lasix bans did not significantly impact the frequency of horses experiencing for exercise-induced pulmonary hemorrhage. [24]

Jockey Club records show the percentage of starts on Lasix in graded race stakes in the U.S. decreased from 98% in 2018 to 11% in 2021. The rate of those horses not finishing races also reduced from 0.52% in 2018 to 0.29% in 2021. [24]

These numbers, however, do not reflect the potential long-term effects of repetitive lung bleeding in horses. And the AAEP still supports the use of race-day furosemide to control EIPH.

Lasix Alternatives

Furosemide is still the most effective treatment for exercise-induced pulmonary hemorrhage in horses. Horse owners should consult their veterinarian to determine if the benefits outweigh the potential risks for their horses. [1]

If your horse races or competes in a jurisdiction that does not allow furosemide, alternative management options may help reduce the risk of EIPH.

Nasal Strips

Nose strips or sasal dilators are a popular management tool for horses that struggle with EIPH. These sticky strips use stiff material applied just below the nasal bone to expand the nostrils.

Nasal strips increases airflow into your horse’s lungs during exercise by opening up the soft tissue of the upper airway. Studies show that nose strips allow horses to take full breaths with less effort, which might reduce pressure on pulmonary capillaries. [25]

More research is neeeded to investigate the effectiveness of nasal strips on lung bleeding, as existing studies have mixed results. [26]

Concentrated Equine Serum

Also called Serumane, Concentrated Equine Serum (CES) contains concentrated immunoglobulins and serum proteins collected from equine blood donors. [27]

Research suggests that CES has immunomodulatory and anti-inflammatory effects on respiratory diseases, which could benefit horses with bleeders. In one study, a CES treatment regimen resulted in a 53% decrease in red blood cells found in broncho-alveolar lavage fluid. [27]

While CES research is promising, this treatment is relatively expensive and may not be accessible to most horse owners.

Omega-3 Fatty Acids

Supplements containing omega-3 fatty acids may also support horses with EIPH by regulating airway inflammation.

Research suggests that the omega-3s EPA and DHA help to preserve red blood cell membrane fluidity during exercise, which supports the pulmonary function. [27] One study reported decreased bleeding in 10 Thoroughbred horses with EIPH after 83 to 145 days of supplementation with DHA and EPA. [27]

Mad Barn’s W-3 Oil is a fat supplement that provides 1500 mg of DHA per serving. Feeding w-3 Oil support healthy regulation of inflammation throughout the horse’s body.

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  • Exercise-induced pulmonary hemorrhage is a common bleeding condition in horses that can lead to poor performance and permanent lung damage.
  • Lasix (furosemide) is an effective treatment for reducing the severity of EIPH in exercising horses, particularly racehorses.
  • Furosemide is a diuretic that increases water loss through urination to lower the horse’s blood pressure. This helps to reduce lung bleeding.
  • Improper use of Lasic can cause dehydration and electrolyte imbalances. Following exercise, horses require electrolyte supplementation and fresh, clean water to support exercise recovery.
  • Controvery exists over the use of furosemide as a race-day medication due to concerns about health risks and performance-enhancing effects.
  • Consult your veterinarian to determine if Lasix is appropriate for your horse. Alternative treatments for EIPH also exist.

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  1. Brewer, K. et al. The Benefits of Lasix for Exercise-Induced Pulmonary Hemorrhage: 50 Years of Science and Clinical Experience. The Horsemen’s J. 2019.
  2. Hinchcliff, K. et al. Exercise Induced Pulmonary Hemorrhage in Horses: American College of Veterinary Internal Medicine Consensus Statement. J Vet Intern Med. 2015. View Summary
  3. Rush, B. Exercise-induced Pulmonary Hemorrhage in Horses. Merck Manual. 2014.
  4. Lequillette, R. et al. Tracheobronchoscopic Assessment of Exercise-Induced Pulmonary Hemorrhage and Airway Inflammation in Barrel Racing Horses. J Vet Intern Med. 2016.
  5. West, J. et al. Stress failure of pulmonary capillaries in racehorses with exercise-induced pulmonary hemorrhage. J of App Phys. 1993.View Summary
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  13. Butudom, P. et al. Rehydration fluid temperature affects voluntary drinking in horses dehydrated by furosemide administration and endurance exercise. Vet J. 2004. View Summary
  14. Fielding, C. et al. Estimation of Acute Fluid Shifts Using Bioelectrical Impedance Analysis in Horses. J Vet Intern Med. 2008. View Summary
  15.  Pritchard, A. et al. Influence of Long-Term Furosemide Use on Bone Mineral Content, Bone Metabolism Markers, and Water Weight Loss in Horses. J Equine Vet Sci. 2019. View Summary
  16.  Freestone, J. et al. Influence of furosemide treatment on fluid and electrolyte balance in horses. Am J Vet Res. 1988. View Summary
  17. Johansson, A. et al. Furosemide Continuous Rate Infusion in the Horse: Evaluation of Enhanced Efficacy and Reduced Side Effects. J Vet Intern Med. 2008.
  18. Houpt, K. et al. Effect of Chronic Furosemide on Salt and Water Intake of Ponies. J Equine Vet Sci. 2016.
  19. Leste-Lasserre, C. Wave of horse deaths on famed racetrack poses puzzle. Science. 2019. View Summary
  20. Bennet, E. et al. Fifteen risk factors associated with sudden death in Thoroughbred racehorses in North America (2009-2021). J Am Vet Med Assoc. 2022. View Summary
  21. Papademetriou, V. Effect of diuretics on cardiac arrhythmias and left ventricular hypertrophy in hypertension. Cardiology. 1994.
  22. Hinchcliff, K. et al. Furosemide reduces accumulated oxygen deficit in horses during brief intense exertion. J Appl Physiol. 1996 View Summary
  23. Barnes, C. Lasix is a “Failed Experiment”: Why the United States Should Follow Other Countries and Ban Lasix. Kentucky J Equine Ag Nat Resour Law. 2019.
  24. Angst, F. Closer Look at Initial Impact of Lasix Phase Out. BloodHorse. 2022.
  25. Kindig, K. et al. Nasal Strips and EIPH in Exercising Thoroughbred Racehorse. J Applied Phys. 2001. View Summary
  26. Goetz, T. et al. Nasal strips do not affect pulmonary gas exchange, anaerobic metabolism, or EIPH in exercising Thoroughbreds. J of Applied Phys. 2001.View Summary
  27. Erickson, H. et al. Review of Alternative Therapies for EIPH. AAEP Proceedings. 2007.
  28. Gross, DK. et al. Effect of furosemide on performance of Thoroughbreds racing in the United States and Canada. J Am Vet Med Assoc. 1999. View Summary