the MDR1 gene). A mutation in ABCB1 prevents P-gp from being correctly produced, so that dogs with this mutation have an increased sensitivity to drugs (such as acepromazine) which are substrates of P-gp. Dogs which are heterozygous (that is, which have one functioning ABCB1 gene, and one non-functioning gene) are less sensitive to acepromazine than dogs which are homozygous (that is, which have two copies of the mutant gene). 75% of Collies carry the mutated ABCB1 gene, as do 50% of Australian Shepherds. Other affected breeds include: Border Collie, English Shepherd, German Shepherd, Old English Sheepdog, and Sighthounds, shelties, long haired greyhound. Tests for this mutation are available. Equine In equine surgery, premedication with acepromazine has been shown to reduce the perianaesthetic mortality rate, possibly due to its actions as a sedative and anxiolytic. It is less effective as a sedative if the horse is already excited. 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. While it is shown to elicit vasodilation in the distal limb, evidence showing its efficacy at increasing perfusion in the laminae is lacking. It is also sometimes used to treat a horse experiencing equine exertional rhabdomyolysis. In the UK, acepromazine is not authorized for use in horses intended for human consumption. Adverse effects Side effects are not common, but the use of acepromazine in stallions should be used with caution (but is not absolutely contraindicated) due to the risk of paraphimosis and priapism. Acepromazine also lowers blood pressure, and should therefore be used with caution in horses that are experiencing anemia, dehydration, shock, or colic. It should not be used in horses dewormed with piperazine. References Alpha-1 blockers Antiemetics Aromatic ketones Dimethylamino compounds Equine medications Phenothiazines Sedatives Typical antipsychotics