A short study on bleeding, for perspective.http://www.scef-inc.com/bayly.html
And, From what i read, bleeding is genrally and most likely caused by high blood pressure forcing blood into the lungs.
Not allergies, or infections or etc, . Sure, it is "performance enhancing" over horses with blood in their lungs! it has side effects, like it has in humans, I suppose; side effects that can be counteracted (short of RETIREMENT!) by diet or supplements.
Dr. Lyons is on an anti-lasix crusade and is preparing a large study involving thousands of racehorses.