The most widely used tranquilizer in horses is acepromazine or “ace.” Few people truly understand its effects. Ace doesn’t make a horse happy, peaceful or calm—it makes him dull and flat.

Ace affects learning. If you use it to make a horse more manageable during schooling you’re wasting your time—he won’t remember most of what you try to teach him.

Ace causes muscular relaxation, decreasing spontaneous motor activity. This means horses that fidget, pace or paw are likely to stop even with relatively low doses. However, you’ll note a lack of coordination and stumbling when a horse is on ace. So, riding a horse on ace is foolish at best.

Ace has side effects. It will cause the penis to drop, which can result in a rare permanent paralysis of the muscles that retract the penis. Heart and circulation are affected by ace, causing a drop in blood pressure.

In the initial stages after administration, irregular heart beat and blocked conduction of impulses through the heart are common.  Therefore using ace on electrolyte-depleted, exhausted, injured, colicked, stressed or in-shock horses is dangerous.

Ace also affects the blood, causing a drop in red-blood-cell counts, hemoglobin and hematocrit.  The effect increases with dosage.

Ace accumulates in the body. If a horse is given ace daily, he’ll eventually need smaller doses for the same effect. Horses receiving ace regularly may test positive at competitions for as long as three weeks after the drug has been stopped.

The main indication for tranquilizers is for the safe performance of medical procedures that can’t be accomplished by other methods of restraint.  In all other cases, the risks and benefits should be carefully weighed. Avoid using tranquilizers for routine tasks, such as pulling manes.  For the handful of horses that go bonkers in certain situations, like in a trailer, either because they can’t overcome their fear of being restrained in a small spot or because of a bad experience, low-dose ace may be necessary in a pinch. 

A horse that has been stall bound for a prolonged period as a result of an injury and is being brought back may be given ace. No one can expect these horses to behave. Proper tranquilization facilitates handwalking and turn-out in small quarters, such as a bull pen, and lessens the chance of injury.

Bottom line: Low-dose tranquilization can help with an immediate, urgent problem but should not be used repeatedly as a substitute for proper training. Consult your veterinarian before using ace.