[QUOTE=CHT;8304183]
No, it isn’t illegal. It is against the rules of showing. Nobody is getting arrested for using a calming supplement like PP.
But people have posted previously that this is abuse, and I am curious of many feel that way, or just feel that it is cheating.[/QUOTE]
Many of these substances are potentially dangerous to the horse (dex can cause horses to founder; magnesium can kill it) or indirectly dangerous when you take an impaired horse over a course of jumps.
Even sticking a horse with a lot of needles unnecessarily isn’t good for it.