Lasix and Blood Viscosity

One of the questions that we are frequently asked is, “What is the effect of Lasix on blood viscosity?”. Bleeds, or exercise induced pulmonary hemorrhages (EIPH), affect the majority of horses during intense exercise, and Lasix is often used to reduce or prevent bleeds.

Lasix (or furosemide) is a potent loop diuretic that increases urine production and urinary frequency. Lasix has been administered to horses before races for the past 40 years as a way to reduce or prevent bleeds [1]. Because Lasix reduces plasma volume, it is believed by many experts to reduce blood pressure in the lungs and prevent bleeds from occurring. Lasix has been shown to reduce plasma volume by 11-13%, and the effect lasts for up to 3-4 hours [2]. Despite the reduced blood pressure and plasma volume in furosemide treated horses, veterinary researchers at the University of Pennsylvania stated, “Using a visual endoscopic scoring system, numerous studies conducted after racing have shown either a slight or no reduction in EIPH in horses administered furosemide before racing” [1].

An authoritative review by equine veterinary scientists Lawrence Soma and Cornelius Uboh published in the Journal of Veterinary Pharmacology and Therapeutics stated that the reduction in blood pressure in the lungs caused by Lasix is not sufficient in magnitude to prevent bleeds [3]. With respect to the improved racing times observed for horses on Lasix, the researchers suggested this benefit may be due to lower body weights of horses on Lasix rather than reduced bleeds.

A separate study performed by researchers at the University of Minnesota showed that blood viscosity and hematocrit (the packed cell volume of blood) were higher in horses that were given Lasix. Viscosity levels were dose-dependent—with greater Lasix doses resulting in higher viscosity levels. Blood viscosity levels of horses given Lasix were higher than those who were not, even after the hematocrit was equalized to 40 in both groups [4].

Another study performed by the same research group at the University of Minnesota examined the in vitro effect of Lasix on blood viscosity and red blood cell deformability. Horse blood samples were collected, and varying doses of Lasix were added to blood in the laboratory bench-top setting. Both the viscosity of blood and the stiffness of the red blood cells were reported to increase [5].

Most veterinarians, trainers, and horse owners have their own view on Lasix. Some research studies demonstrate that Lasix is effective for reducing bleeds, while other studies show conflicting results. Putting aside the debate and controversy, it is clear that Lasix has an effect on the blood and the cardiovascular system.

Blood viscosity testing is the only direct way to measure the ability of blood to flow. We offer the Equine Blood Viscosity Test to allow veterinarians, trainers, and owners to screen and monitor blood flow in horses. By measuring systolic and diastolic blood viscosity, we are able to determine the thickness and stickiness of blood. Blood viscosity measurements can help detect blood flow abnormalities and may help track the effectiveness of treatments for EIPH.


1. Birks EK, MM Durando, McBride S. Exercise-induced pulmonary hemorrhage. Vet Clin North Am Equine Pract 2003; 19:87-100.

2. Hinchcliff KW, McKeever KH, Muir WW 3rd. Furosemide-induced changes in plasma and blood volume of horses. J Vet Pharmacol Ther 1991; 14:411-7.

3. Soma LR, Uboh CE. Review of furosemide in horse racing: its effects and regulation. J Vet Pharmacol Ther 1998; 21:228-40.

4. Geor RJ, et al. Effects of furosemide and pentoxifylline on blood flow properties in horses. Am J Vet Res, 1992; 53:2043-9.

5. Weiss DJ, Evanson OA, Geor RJ. The effects of furosemide and pentoxifylline on the flow properties of equine erythrocytes: in vitro studies. Vet Res Commun 1994; 18:373-81.