In fact, I spoke to a middle schooler over the weekend that mentioned she had given her horse a "happy drug" when he was being particularly high-strung. I got curious and looked up the rules for the local circuit and it states that they follow USEF rules. Those rules are a little bit hard to follow but as far as I could tell, it's illegal and it's a fineable offense.
It got me curious so I looked it up in AQHA and was surprised to see that it IS legal but then when I read further it said IF the horse has been diagnosed with HYPP. So I'm assuming it's used as a treatment for the disease, that I found interesting, I had no idea.
Now I used this drug on my mare when she severed her tendons and we were pulling her cast to change her bandages daily. It was REALLY painful and she got really good at kicking! The vet had told me that it was a relatively safe drug but that it effected the...hmmm... neourological system I thought, which to me, would make me think the horse could potentially be dangerous to ride when under the influence... Am I mistaken?
Also the vet had said they don't really have a good recall of what happened when they were under the influence, which to me, would mean that if you took a horse that was new to a situtation, and drugged it, then it would not recall any of it and the entire training/situation was a waste. And to me, it seems like it would be cheating in the show ring.
What are others thoughts? Have you used it to show? Do you believe it's a fairly common thing? Is it most prevailent in the hunt ring? I'm just curious....