Many people use furosemide (Salix, Lasix) routinely to help prevent bleeding in horses suffering from EIPH (Exercise Induced Pulmonary Hemorrhage). In most cases, a dose of furosemide is given and water is withheld at least 4 hours before competition in order to prevent bleeding. The theory is to decrease blood pressure and help prevent bleeding.
Like most medications, furosemide has many other side effects. It has long been considered a performance enhancing drug in the racing industry. One potential reason for this is the effect it has on body weight. A lighter horse expends less energy so takes longer to fatigue. Tests have shown that treated horses expend less energy and have less lactate build up.
So how much weight does a horse lose with treatment? You might be surprised. Researchers found that when feed, hay and water were withheld for 4 hours, untreated horses lost an average of 9 pounds. Horses treated with 150mg (3cc) furosemide lost 28 pounds, those treated with 250mg (5cc) lost 31.5 pounds and those treated with 500mg (10cc) lost 32 pounds.
Common sense says to give these horses free access to water ASAP after completion to replace this loss. The problem is– tests also show treated horses did not drink any extra water over untreated horses over the first 20 hours. This means many of these horses are below normal hydration at 24 hrs. One study showed that it can take up to 3 DAYS for the body to completely regain lost fluid weight from one injection of furosemide. (Add to this when people give Lasix 3 or 4 days in a row!)
Furosemide works by decreasing sodium absorption in the kidneys and interferes with calcium and magnesium transport. This decrease in sodium then leads to excreting more dilute urine. Research showed sodium and chloride losses in the first 4 hours are as high as 40-50 times that in a normal horse. Calcium loss was increased for up to 72 hours after a single injection. Potassium was unaffected.
So from one injection you may see initial performance improvement from preventing (or decreasing severity) of bleeding and from weight loss. This is then countered by a horse that will have some altered sodium, chloride and calcium losses and be somewhat dehydrated for up to 3 days.
Mineral losses are easily replaced with proper electrolyte supplementation. Regaining fluid is the key because even the 1.5-2.5% dehydration levels noted in the research can cause fluid shifts in the body. These shifts can result in loss of fluid in the intestinal tract which can lead to colic. KERX has developed a two stage electrolyte program called RACE RECOVERY. This powder and paste were designed to increase water intake and replenish minerals. Their studies showed a 17% increase in water intake and a 30% improvement in body weight rebound.
If furosemide is needed to prevent a medical problem then it must be used. Other medications (bronchodilators, etc) and herbals can also be used but may or may not be as effective. The key is to remember even low doses of furosemide can have huge effects. These can be compounded with hot weather AND multiple day events. Many people give the electrolyte pastes to try to help replenish minerals. This is good but the horse still must drink more water than normal over a few days to put the system back in normal condition.
Most of the information above came through research conducted by Kentucky Equine Research (KER). Most tests cited were performed on thoroughbred and Standardbred race horses.