Nearly all of the horses in running in the Kentucky Derby will be given a drug on Derby day that is banned in most of the rest of the world.

The drug is called Lasix, a powerful diuretic that is used to prevent race horses from bleeding that many wonder if it is performance enhancing. Outside the United States, Lasix is often permitted for training but not given on race day because of its unclear effect on performance. However, veterinarians and many trainers support the use of Lasix in race horses because it prevents damage to the horses’ lungs caused by a spontaneous bleeding condition.

“Lasix is a preventive medicine, it prevents damage to the horses’ lungs,” James Casey, a veterinarian on the board of the North American Association of Racetrack Veterinarians, tells Inverse. “It’s not performance enhancing, it’s performance enabling. It allows a horse to compete to its natural ability.”

Racehorses often suffer from a condition called “exercised induced pulmonary hemorrhage,” Casey says. This is when the tiny capillaries inside a horses lungs burst because of the stress of having 60-70 gallons of blood a minute pumping through them. Some horses bleed more than others, but it’s a condition that gets worse over time. For Casey, using the drug is a no-brainer, because it prevents any damage to the lungs from getting worse.

Critics of Lasix point out that it is unclear if the drug hides performance-enhancing substances that could be present before a race, or that the weight-loss caused by its diuretic effect is performance enhancing itself. It’s banned in the rest of the world on race-day because of this, and because bleeding is seen as a sign of other problems in the horse. But Casey points out that there simply haven’t been studies to actually determine the effect of Lasix on performance.

As for hiding things like anabolic steroids, Casey says he’s not concerned. “The rules have not caught up with the testing methods,” he says. “Trace amounts of [legally given] therapeutic medications can be found in post-race tests which can be a big problem.” In other words, Casey thinks that drug-testing methods are so sensitive that even with the use of Lasix they will catch medications like penicillin long after the active period of the drug.

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In the end, the problem with Lasix is that it does have a powerful therapeutic use that can help horses. But many say only five percent of racehorses really suffer this condition, making Lasix’ widespread use problematic, particularly because it isn’t clear what else it does. In people, diuretics are banned before competitions because they cause weight loss right before a match and can be used to mask the presence of other performance enhancing drugs. But until there’s a way to prove that happens in horses, Lasix is likely to be the drug of choice for your favorite to win the Kentucky Derby.

Photos via Getty Images / Andy Lyons