Hi there!
Pardon me for butting in, but I can see why they might ban it. If it's a vasodialator, it might enable some horses blood streams to carry more blood (oxygen) to their muscles, thereby giving them an unfair advantage, kind of like "Blood Doping" does for Olympic Athletes. Back in the 2002 Winter Olympics, two cross country skiers were stripped of their medals for doing it.
Looks like you're right about that.... I found it banned in other disciplines.
Mack just pointed out a good point. Take the use of this drug in context. If a fully conditioned, young, athlete takes it, that's one thing, but a 22 year old navicular horse???? Big difference.
Becky