SI Vault
Clive Gammon
August 10, 1987
What exactly is Lasix?
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August 10, 1987

What The Medics Say

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What exactly is Lasix?

It's the brand name under which furosemide, a potent diuretic, is marketed in the U.S. Also used to treat humans, furosemide increases urination and thereby reduces edema—internal accumulation of fluids—and lowers blood pressure. But when Lasix was first encountered by American horsemen in the early '70s, the drug was hailed not so much for these clinically established qualities as for its reputed effect on what veterinary scientists call Exercise-Induced Pulmonary Hemorrhage, or EIPH, the cause of which is still largely unknown.

Horsemen know EIPH simply as "bleeding," a classic example of which occurred in this year's Kentucky Derby when Demons Begone bled profusely from the nostrils as he reached the backstretch and had to be pulled up. Horses breathe almost exclusively through their nostrils. If the lungs and nostrils are congested with blood, choking ensues. Lasix appears to reduce bleeding, most likely by lowering a horse's blood pressure—though veterinarians aren't certain. At the same time, the general reduction of fluids in the animal's body tends to enlarge the respiratory area of the trachea, facilitating breathing.

The drug also appears to enhance racing performance. Dr. George A. Maylin, a Cornell University veterinarian, says, " Lasix is the only drug that I am aware of, of which it has been so conclusively demonstrated that it improves performance. There is no data for amphetamines, no data for etorphine, that suggests these drugs can improve performance like Lasix."

In a seminal study published in April 1985, a research team led by Dr. L.R. Soma of the University of Pennsylvania's School of Veterinary Medicine analyzed 128 thoroughbreds then running in Pennsylvania and Maryland. Three groups of the horses were known to be affected by EIPH in various degrees; the fourth was a control group of nonbleeders. Their performances were followed over 10 races. In the first five the EIPH groups went untreated; in the last five they were injected with Lasix. Group 4 was untreated throughout. Results were analyzed according to average finishing positions and time. Two of the EIPH groups showed significant improvement on Lasix. In Race 6, with the first use of Lasix by the EIPH horses, improvement was particularly sharp. The control group showed no significant performance change.

It is unclear whether it is only reduced bleeding and enhanced breathing that improves a horse's performance, or whether Lasix also provides some kind of additional boost, or "lift," as horsemen put it. There have been no studies yet on the impact of Lasix on nonbleeders, which might indicate the drug's secondary effects.

What is certain about Lasix is that it is a diuretic. Normally, says Maylin, a horse might urinate an average of one-half liter in an hour. With Lasix, that rate can go up to 20 liters per hour during peak effectiveness—usually within two hours of injection of the drug. Of concern to many in racing is the fact that Lasix, by increasing urination, can dilute, and thus effectively mask, other—illegal—drugs that might be given a horse before a race.