Lasix is a highly debated/ hot topic in the barrel racing world. It seems that people either think everyone should have their horses on it or they are very against it, so lets explore it and see what we find.

Reasons why Veterinarians recommend Lasix:

 1. To Prevent or Reduce Bleeding in the Lungs (EIPH)

What is EIPH?

During intense exercise (like racing), some horses experience very high blood pressure in the lungs. This can cause small blood vessels to rupture, leading to bleeding into the airways. The condition is called Exercise-Induced Pulmonary Hemorrhage (EIPH) and is well documented in performance horses.
(Source: American Association of Equine Practitioners)
https://aaep.org/issue/exercise-induced-pulmonary-hemorrhage-eiph

What Lasix does:

Lasix (furosemide) works primarily as a diuretic, meaning it causes the horse to urinate and lose fluid. This temporarily lowers blood volume and reduces pressure in pulmonary blood vessels, which can decrease the severity of lung bleeding during intense exercise.
(Source: JAVMA research on furosemide and EIPH)
https://avmajournals.avma.org/view/journals/javma/235/1/javma.235.1.76.xml

• Acts as a diuretic (makes the horse urinate)

• Reduces overall blood volume

• Lowers pressure in lung blood vessels

• Decreases the severity of bleeding

Not all horses with EIPH show visible nosebleeds — many cases are only detected via endoscopy after racing or strenuous exercise.
(Source: Merck Veterinary Manual)
https://www.merckvetmanual.com/respiratory-system/respiratory-diseases-of-horses/exercise-induced-pulmonary-hemorrhage-in-horses

After scoping and seeing signs of bleeding, many veterinarians recommend Lasix to help reduce the severity of bleeding during future performance events.

However, Lasix is not a long-term fix.

Why It Can’t Be a Cure

Think of it like this:

If a pipe bursts because water pressure is too high…

Lasix = turning down the water pressure before turning the faucet on.

It does NOT:

• Replace the weakened pipe

• Reinforce the pipe

• Change the plumbing system

If you remove the pressure control, the problem can happen again.

 Why Long-Term Use Doesn’t “Fix” It

Repeated EIPH can cause:

• Chronic lung remodeling

• Hemosiderin accumulation

• Fibrosis

• Reduced oxygen exchange efficiency

These long-term changes have been documented in research on racehorses with repeated EIPH episodes.
(Source: Veterinary Clinics of North America – Equine Practice)
https://www.sciencedirect.com/science/article/abs/pii/S0749073914000703

Lasix does not stop long-term remodeling — it simply reduces the severity of bleeding during a specific event.

Even when bleeding is reduced, the underlying structural stress on lung capillaries may continue over time.

 2. It Reduces Body Fluid and Weight

Because Lasix causes fluid loss:

Horses can lose 20–30 pounds of water weight following administration.

Research has shown that this weight reduction can slightly improve race performance.
(Source: PLOS ONE study on furosemide and race performance)
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0137023

This performance effect is one of the reasons Lasix use has been controversial in racing.

As people continue giving their horses Lasix, a common thought can be:

“If it works, increasing the dose should work better.”

More Is Not Always Better

Higher doses increase risk of:

• Dehydration

• Electrolyte imbalance (especially potassium loss)

• Muscle cramping

• Performance instability

• Kidney stress

These side effects are well documented with diuretic use in horses.
(Source: Equine Veterinary Journal – furosemide pharmacology)
https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.12387

There is a point where increasing the dose does not improve pulmonary protection.

What Research Shows

Most studies show:

• Standard weight-based dosing is effective

• Higher doses do not proportionally improve protection

• There isn’t strong evidence of progressive “tolerance” requiring continual escalation

If doses keep rising, it usually signals:

• Worsening EIPH

• Inadequate overall management

• Or lack of re-evaluation

 3. Common in Racehorses

In many racing jurisdictions (especially historically in the U.S.), Lasix has been widely used on race day. However:

• Some regions have restricted or banned race-day Lasix

• Especially in 2-year-olds and major stakes races

Why Race-Day Lasix (Furosemide) Is Banned or Restricted on Some Racetracks:

1. International Medication Policy

Many racing jurisdictions outside the United States operate under a “no race-day medication” policy to maintain drug-free competition standards.

(Source: International Federation of Horseracing Authorities)
https://ifhaonline.org/resources/ifAgreement.pdf

2. Performance Effects Beyond Therapy

Furosemide can reduce plasma volume and body weight through diuresis, which may provide a performance advantage separate from its medical effect on EIPH.

(Source: Sullivan et al., PLOS ONE study)
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0137023

3. Drug Testing Integrity

As a powerful diuretic, furosemide can dilute urine and plasma samples, which historically raised concerns about masking other prohibited substances.

(Source: Racing Medication and Testing Consortium)
https://rmtcnet.com

4. Welfare & Public Perception

Regulators have also argued that if a horse requires medication to compete, it raises questions about welfare and public confidence in the sport.

Recent policy reforms aim to align North American racing with international standards.

(Source: Horseracing Integrity and Safety Authority)
https://hisaus.org

Why Lasix is commonly chosen?

• • It reduces blood volume
  • That lowers pulmonary vascular pressure
  • Lower pressure means less stress on fragile lung capillaries
  • Result: Less bleeding during high-speed exercise

Research has consistently shown that it can reduce the severity of EIPH episodes during high-intensity exercise.
(Source: JAVMA EIPH study)
https://avmajournals.avma.org/view/journals/javma/235/1/javma.235.1.76.xml

However:

Lasix does not cure EIPH.

It only reduces the severity of bleeding during that specific event.