Furosemide, also known as Salix, Lasix and Furoject, decreases the incidence of haemorrhage, according to the results of the study.
The research, conducted by Colorado State University, the University of Melbourne and the University of Pretoria in South Africa, provides a foundation for racing authorities to make decisions regarding the use of this medication, which is the subject of heated debate and controversy around the world.
Although furosemide has been used in the racing industry for several decades, no scientifically sound studies have been conducted to prove or disprove an effect on airway bleeding.
The study involved 167 horses randomly allocated to race fields of 9 to 16 horses each.
Each horse raced in two races, one week apart, in the same field and in races of the same distance.
In the blinded study, each horse received furosemide before one race and saline solution before the other race. Horses raced under typical racing conditions.
Endoscopy examinations were performed within 30 to 90 minutes after racing to identify the presence of blood in airways. Full details of the study will be released in the Journal of the American Veterinary Medicine Association on July 1.
The research showed that giving furosemide before a race dramatically decreased the incidence and severity of exercise-induced pulmonary hemorrhage, or EIPH.
Horses were three to four times more likely to have any evidence of bleeding without furosemide, and were seven to 11 times more likely to have severe bleeding without it.
EIPH is the medical term for spontaneous bleeding that occurs within a horse's airways and lungs during exercise.
"The results of this study do not eliminate debate about the use of this medication in racehorses, but it does provide evidence needed to aid making sound policy decisions," said Dr Paul Morley, one of the principal investigators of the study and a veterinarian at Colorado State University.
"Decisions are always easier when you have data."
Dr Kenneth Hinchcliff of the University of Melbourne and Dr Alan Guthrie of the University of Pretoria, also veterinarians, were the other principle investigators in the study.
"We designed this study to provide the highest quality evidence to address the use of furosemide in thoroughbred racehorses," Hinchcliff said.
"This study design was similar to those used to test the efficacy of treatments in human medicine, which, to date, have been uncommon in evaluating treatments for horses."
Furosemide is widely used in the horse racing industry in North America but is banned on racedays in all other countries. More than 90 per cent of racing thoroughbreds and 50 per cent of racing standardbreds in the United States and Canada are given furosemide a few hours before racing to treat bleeding.
However, despite this common practice, before this study there was no conclusive evidence that furosemide was effective in preventing or limiting lung bleeding in racehorses.
Use of furosemide to treat pulmonary haemorrhaging in racehorses began in the 1970s.
Today it is estimated that the racing industry spends about $US30 million annually to treat thoroughbred horses with furosemide on race days in the United States and Canada.
Use of the medication is controversial because work previously conducted by these scientists found that it enhanced the performance of throughbred and standardbred horses. Treatment is also controversial because some critics say that its use confirms animal welfare problems associated with horse racing.
Because of their unique physiology, all horses running at racing speeds experience varying degrees of exercise-induced pulmonary hemorrhage, or bleeding into their airways.
Because of blood pressure changes in the lung that are unique to horses during exercise, more than half of thoroughbred racehorses have small amounts of blood in their trachea after a single race.
While severe EIPH is uncommon, this same research group confirmed the widely held belief that bleeding into the airways impairs athletic performance of horses.
Other than the use of furosemide, which was administered strictly adhering to research project guidelines, all races were under South Africa's standard rules and regulations for the industry.
The horses and jockeys raced for purses to ensure competitive racing efforts. The five to eight furlong (1000m-1600m) races were on a one-mile straightaway on the turf at the Vaal Racecourse in South Africa.
Furosemide is a diuretic - it reduces body fluids by increasing urination. It is used in many species, including humans, to control blood pressure and fluid balance.
"Support for this study from the racing industry in South Africa and the US was tremendous," Guthrie said.
"The Grayson-Jockey Club Research Foundation and the Racing Medication and Testing Consortium, both from the United States, and Racing South Africa and the Thoroughbred Racing Trust from South Africa provided grant support for this research. Numerous private donors included high-profile private sponsors, such as golfing legend and horseracing enthusiast Gary Player, provided monetary support, Phumelela Gaming and Leisure donated the use of The Vaal racetrack with all of its personnel and facilities, The National Horse Racing Authority of Southern Africa provided its staff to officiate at the races and the local trainers and owners enthusiastically allowed use of their valuable horses in this unique study."