I thought this was a very good artical on Ace.

"There are four big sedatives that a vet uses for average horses: Ace, Dormosedan, Torbogeisic, and Rompun. Ketamine can be used also but that is usually just for sedation prior to surgery. Of those drugs, Ace is the only one that is not a controlled substance. The other drugs require that the vet record how much was given and to whom.

The reason Ace is so readily handed out is because it is not a controlled sustance, and compared to the others, it is the safest for the common horse owner to use. Torb, Dormosedan, and Rompun are too easy to overdose, and are very dangerous if owners accidently pokes themselves. Ace requires a large dose to accidently kill the animal, and it mostly just burns when a person gets stuck by accident.

Ace is a Vesodialater. In other words it opens the blood vessels and drops the blood pressure, that is how it causes a sedating effect. In addition to sedation, that can increase blood flow. So if you have an injured horse, or one that is foundering - a vet may suggest Ace to not just take the edge off the pain, but to increase blood flow to the affected area. It works really well for founder cases.

There are of course bad things that can happen with Ace, but that can be said for any medication. And while I think training is a better alternative, but sometimes you need the horse to calm down and you don't have time for training. And Rompun will cause the penis to drop also - in fact that is one of the benefits to it, but it is very dangerous in the wrong hands so Ace is given as the lesser of two evils."