Written by Dr. Warwick Bayley
Washington State University, Professor, Equine Medicine
The use of furosemide (also known as Salix® and Lasix®) in racehorses is controversial. It is permitted during workouts and may be administered for racing within certain regulatory frameworks.
While it has been scientifically demonstrated to reduce the severity of bleeding associated with Exercise-Induced Pulmonary Hemorrhage (EIPH), concerns remain that it may also enhance performance. Regulatory entities continue to evaluate the use of furosemide on race day.
Performance Enhancement: A Complex Definition
A search on the internet reveals a variety of definitions of “performance enhancement” and multiple categories of performance-enhancing substances. However, what constitutes a “performance-enhancing substance” is not always clear-cut.
Although it is generally agreed that any agent taken to increase strength, endurance, recovery, and other physical and psychological abilities beyond what is considered natural for the individual, in order to gain an unfair competitive advantage, should be banned, some drugs such as caffeine continue to be used by human athletes.
Diuretics and Anti-Doping Regulations
Virtually all testing for prohibited substances in horses now relies on testing of blood samples, which has largely alleviated concerns regarding the potential role of furosemide as a masking agent.
Diuretics, including furosemide, are banned by the World Anti-Doping Agency (WADA) because they have been used by athletes to excrete water for rapid weight loss (generally to make a weight limit in a particular sport) and to mask the presence of other banned substances by inducing the production of large amounts of dilute urine, which lowers the concentrations of other banned agents in the urine and makes them harder to detect.
This latter point is the primary reason that diuretics are banned in human athletes and, in the past, has been a major concern in horses too. However, because of advances in analytical chemistry, virtually all testing for prohibited substances in horses now relies on testing of blood samples, which has largely alleviated concerns regarding the potential role of furosemide as a masking agent.
There is scientific evidence that horses treated with furosemide four hours before racing or exercising at high speed to fatigue on a treadmill do perform better than might have been the case had the drug not been administered, and that this improvement is also related to the weight loss associated with the diuresis. A similar effect might be achieved with judiciously limited access to water in the 2-3 days leading up to a race, as is practiced in camel racing in the Persian Gulf region.
Health Implications for Horses versus Humans
There are, however, important differences in the health and welfare implications of furosemide-induced weight loss in humans and horses. Whereas some people, including jockeys, have died because of a combination of dehydration and electrolyte disturbances associated with the use of diuretics, these problems do not appear to manifest in horses when they race.
Part of the reason for this might be the regulated period of time (four hours or more) between when furosemide can be given and when the horse races, the maximum allowed dose of the drug, and the capacity of the horse’s large intestine or colon to serve as a reservoir for water and electrolyte replacement in the face of their sudden loss from the body for any reason.
Abattoir studies have documented that there is about 25-30 liters of fluid in most equine large colons, and this can be readily absorbed to replace fluid lost from the circulation and cells of the body. Consequently, there is good evidence that four hours after receiving furosemide, horses are not dehydrated, and their circulating blood volume and blood electrolyte concentrations are not different from what they were prior to receiving furosemide.
They have, however, lost 1-2% of their body weight, and the moisture content of their manure has dropped by about 3%. This weight loss is the primary reason they perform better after receiving furosemide, as replacing this weight loss has been shown to negate any associated improvement in performance.
“There is emerging evidence that the 3-4% of horses with severe EIPH have shorter careers and fewer races than those in which EIPH is mild to moderate.”
The Dual Role of Furosemide in EIPH Management
In addition to these recognized effects of furosemide, there is another important factor to consider when considering the controversy that affects its use in racehorses. That pertains to the ability to reduce the severity of exercise-induced pulmonary hemorrhage (EIPH) in many horses that exercise/race at top speed. There is an increasing acceptance that some degree of EIPH is an occupational hazard of being a racehorse and that most horses bleed from the lungs to varying degrees sooner or later and with increasing frequency over the course of their careers.
In some cases, the volume of hemorrhage is sufficient to interfere with the horse’s ability to perform. The same cannot be said with respect to human athletes. While the severity of EIPH varies between and within horses from race to race, there is published evidence that the odds that a horse will incur less severe EIPH following pre-race administration of furosemide are significantly greater than the same outcome if it does not receive furosemide. In these instances, the reduction in severity of EIPH might also be associated with an improvement in a horse’s performance.
There is also emerging evidence that the 3-4% of horses with severe EIPH have shorter careers and fewer races than those in which EIPH is mild to moderate. All of these apparently beneficial pulmonary health effects of administering furosemide must be weighed against the improvement in performance that is associated with this same usage, when considering what rules are in the best interests of the racehorses and the industry of which they are the most important component.
Ethical and Regulatory Considerations
Finally, and again with respect to the debate regarding the performance enhancement attributed to the pre-race administration of furosemide, WADA’s understandable definition of the illicit use of performance-enhancing substances refers to the goal of the user gaining an unfair or unnatural competitive advantage. Surveys of furosemide usage in Thoroughbreds between 2000 and 2020 indicate that >90% of horses raced after being injected with the drug. Under such circumstances, it could be argued that the issue of gaining an unfair advantage is moot because 18-19 of every 20 horses racing the same drug have received furosemide.
When such a preponderance of runners has been treated with the drug, how can any of them be gaining an unfair advantage? This, coupled with the fact that the pulmonary health of some of the recipients might also benefit from receiving, further muddies the waters when trying to determine the most appropriate regulations governing the pre-race use of furosemide in America’s racehorses. Objective minds agree that the decision is not a straightforward one.
The Horseracing Integrity & Safety Act has mandated and is funding research into the appropriate use of furosemide as a race day medication. Results of this research are expected to help inform definitive recommendations regarding furosemide-use policies in 2026. Currently, under the HISA Anti-Doping & Medication Control and Safety Programs, the ban on furosemide only applies to 2-year-old races and stakes races. Whether this ban is expanded, rescinded, or changed in any other way, remains to be seen.
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