The biggest debate in American horse racing is over the use of the race day medication, Furosemide, more commonly known as Salix or Lasix. This is the second and final article on the topic of Lasix use in North America; we have previously discussed Exercise Induced Pulmonary Hemorrhage, also known as EIPH or “bleeding” in an earlier article. More information about EIPH can be found here.

Furosemide is a commonly used human medication. It is likely one that you have heard of or perhaps even used first hand. The medication, a diuretic, is used to manage excess fluid in the body and is primarily seen in the treatment of heart failure and kidney disease. Furosemide works by inhibiting the tubules in the kidneys from absorbing water and sodium, thereby increasing urine output dramatically and lowering the overall fluid volume in the body. For clarification and uniformity, “Lasix” is what we will use to describe the medication as that is the trade name most commonly used in the horse racing industry.

It is believed that Dr. Alex Harthill first administered the medication to Northern Dancer prior to the 1964 Kentucky Derby. At that time, EIPH was even less understood than it is now and there was no research into medications that could manage the bleeding. Veterinarians such as Harthill took it upon themselves to use human medications to see if the bleeding could be managed. Eventually, Lasix was legalized in horse racing in the mid-1970s starting with Maryland and was in every racing jurisdiction in 1995 when New York finally allowed the medication to be used. The popular race-day medication has no competition as it is used on a minimum of 95% of racehorses in North America.

The horses who will receive the medication are administered between 5 and 10 mL, depending on the vet and the racing jurisdiction, and it is given a minimum of four hours prior to post time. The nature of Lasix is that it is diuretic that increases urine output dramatically within minutes of its intravenously (into the jugular vein) administration. Lowered blood pressure is a byproduct of the excess urine being excreted due to the mechanics of the medication; this lowered blood pressure then reduces the stress that the pulmonary and cardiovascular systems experience when the horse is at work. The reduction in stress that the delicate capillaries in the lungs experience reduces the occurrence or severity of bleeding. Numerous studies have shown that Lasix is highly effective in reducing the severity of a bleeding episode or perhaps eliminating the occurrence altogether.

The United States and Canada are the only racing entities that allows race-day Lasix. Some countries such as Australia, Japan, France, the United Arab Emirates, and the United Kingdom allow Lasix to be used in training but there are a variety of withdrawal times ranging from two to ten days prior to the horse’s race. Germany and China have banned the use of Lasix altogether, including for training purposes.