Acepromazine can be administered by the intramuscular route, taking effect within 30–45 minutes, or may be given intravenously, taking effect within 15 minutes. Sedation usually lasts for 1–4 hours, although some horses may feel the effects for up to 24 hours. The standard dose is highly variable, depending upon the desired effect following administration. An oral gel formulation is also available (Sedalin gel). The dosage by this route is also highly variable, but it is generally accepted that the recommended dose will give moderate sedation in most horses.
In the UK, acepromazine is not authorised for use in horses intended for human consumption. In equine surgery, premedication with acepromazine has been shown to reduce the perianaesthetic mortality rate, although the reasons for this are unclear.
Additionally, acepromazine is used as a vasodilator in the treatment of laminitis, where an oral dose equivalent to "mild sedation" is commonly used, although the dose used is highly dependent on the treating veterinarian. It is also sometimes used to treat a horse experiencing Equine Exertional Rhabdomyolysis.
Precautions when using in horses
Acepromazine is a prohibited class A drug under FEI rules, and its use is prohibited or restricted by many other equestrian organizations. It can be detected in the blood for 72–120 hours, although repeated doses may make it remain present for several months.
Side effects are not common, but the use of acepromazine in stallions is usually considered contraindicated due to the risk of paraphimosis and priapism.
Acepromazine should not be used in horses dewormed with piperazine. It lowers blood pressure, and should therefore be used with caution in horses that are experiencing anemia, dehydration, shock, or colic.
More or less what it says in plumb's veterinary drug handbook.
It does not effect fertility. Too bad it won't permanently affect temperament...
EDIT: I have more commonly seen Xylazine used in large animal medicine. Just another option.
We drink to our youth, to the days come and gone; for the age of oppression is now nearly done.