New Furosemide Research Reveals Unexpected Impacts Of The Medication by Natalie Voss|06.29.201506.30.2015|4:04pm3:16pm It's no secret that furosemide (Lasix, Salix) is both effective in preventing or lessening exercise-induced pulmonary hemorrhage, or that the drug has been shown to make horses lose weight through dehydration—a side effect many people believe makes the substance attractive as a performance enhancer. Scientists at Kentucky Equine Research wanted to learn more about the drug's dehydration impacts, and released a series of studies last month revealing new information about the amount of weight loss taking place after a dose of furosemide, and what's going on with the equine body as a result. In one study, scientists administered different amounts of furosemide to six Thoroughbreds and measured the amount of weight loss for four hours after each animal's treatment (four hours being the minimum time between administration and race time in most states). To their surprise, they found the amount of weight loss did not vary significantly between treatments; A 150-milligram (about 3 cc) dose caused horses to lose an average of 28 pounds, but even a 500-milligram (about 10 cc) dose of furosemide resulted in a loss of 32 pounds. This seemed to be because the dehydration from a larger dose signaled horses receiving 500 milligrams of furosemide to defecate less. “That was surprising,” said Dr. Joe Pagan, president of Kentucky Equine Research. “The way Lasix works is it interferes with the transporter for sodium. The thing is, once you've saturated that transporter, you can't saturate it any more. I think what that shows is that even a 3 cc dose is pretty much saturating that transporter entirely.” Researchers also discovered that horses do not apparently feel the need to drink more water after having been treated with Lasix (regardless of dose), which means they also don't compensate for the dehydration on their own within the 24 hours after administration. This was true whether or not the horses were exercised following furosemide administration; exercised horses drank more than unexercised horses, but not enough to make up for the extra losses due to furosemide. In fact, it took medicated horses 72 hours to return to pre-furosemide weight. “I would have guessed before doing this that since they'd lost three or four gallons in their urine, they'd be really thirsty,” said Pagan. “I think the reason for that is that thirst has to be signaled by something … usually because you lose water from your blood but sodium stays high.” That doesn't happen after furosemide administration – sodium follows the water out of the horse's system, so there's nothing to tell him he needs to begin hitting the water bucket. KER's studies also revealed that horses treated with furosemide lost about four times more sodium in 24 hours after dosing, and also suffered losses of chloride and calcium during this period. (Oddly, potassium levels remained stable regardless of furosemide dose.) Most horses' diets are adequate to replenish those minerals within a few days following a race on furosemide, Pagan guessed, which means that this dehydration and mineral loss is unlikely to have a long-term effect on a horse's health. He did voice concerns about whether the calcium losses could add up if a racehorse did not have a calcium-rich hay source, however. Furosemide, otherwise known as Salix or Lasix “The very first study we did with Lasix, we analyzed the hay we were feeding, which was just a racetrack timothy hay, and found it actually had more phosphorous than calcium in it,” said Pagan. “The book values say it should be about a one to one ratio. “There may be some horses that are sort of borderline [on calcium levels] anyway, that this [loss from Lasix] could have an effect.” KER engineers used the information from these studies to create a new electrolyte supplement to aid in post-race recovery, both by replenishing mineral levels and triggering horses to drink water. Although the results are initially unsettling, Pagan believes these studies are ultimately “good news.” “I considered it a really good news study,” he said. “Yeah, there are these losses but they can be quickly replaced. As polarized as the world is about Lasix, I hate it that they take our data and say this either proves that Lasix is good or Lasix is bad. It doesn't prove either. It shows that the mechanism of action of Lasix loses a lot of electrolytes, but they're easy to put back.” Still, he's not sure that most trainers or owners are aware that furosemide has an impact beyond increased urination, and he's hoping that will change. “I expect that if you did a poll of trainers asking 'What does Lasix do?' they will tell you the horses pee a lot and they lose weight, but I would venture to say that the vast majority of them would not know how that urine is produced, or that the reason it's produced is that you're actually losing electrolytes first, not urine,” said Pagan. “Nobody, to my knowledge, had quantified the actual amount of what was lost.” Find a full copy of KER's research summary here.