The Case Against the Use of Acepromazine in Male Horses

1. Introduction
Acepromazine is known to cause penile prolapse in male horses, which is why it is frequently administered to facilitate cleaning the sheath. A study of acepromazine in geldings showed that the degree and duration of penile protrusion is dose dependent, with 0.01 mg/kg, IV (~5 mg in a 500-kg horse) causing 40% protrusion of the penis for <30 min, 0.04 mg/kg, IV (~20 mg in a 500-kg horse) being associated with 80% protrusion of the penis for ~1 h, and 0.1 mg/kg, IV (~50 mg for a 500-kg horse) being associated with 90% protrusion for nearly 4 h.1 Other phenothiazines such as chlorpromazine and propriopromazine have also been reported to cause penile paralysis.²  Unfortunately, in rare cases, penile prolapse can result in permanent paraphimosis, particularly if the penis is allowed to become edematous or congested or develops a hematoma.³  This condition can lead to serious and permanent consequences, most particularly in breeding stallions. It has been speculated that the presence of circulating testosterone at the time of acepromazine administration might significantly increase the risk of priapism.⁴  Even stallions that have been previously tranquilized with acepromazine without problem may develop either flaccid prolapse of the penis or priapism on a subsequent administration of acepromazine. The exact mechanism for this complication is unknown, although it has been speculated that α-adrenergic nerve fibers supply motor innervation to the retractor penis muscle, and therefore, the α-adrenergic blockade induced by acepromazine blocks this motor function.⁵  Treatment is supportive: protecting the penis from injury and preventing or minimizing swelling.^4,5  There is one report of successful treatment by administration of the anticholinergic agent benztropine to a gelding with priapism in which only the corpus cavernosum penis, not the glans penis, was engorged.6 In severe cases where the penis remains prolapsed, amputation may be required.³ [...]