The debate about whether thoroughbreds should be treated with a diuretic on the day of a race became thornier Monday when researchers released a study showing that furosemide, known as Lasix, significantly reduces bleeding in horses’ lungs. Furosemide has been used to treat racehorses since the 1970s. Most countries ban race-day use of Lasix because it improves performance. In the United States, however, virtually every horse receives it on the day of the race. “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...