Which drug is contraindicated in breeding stallions due to penile prolapse and lowers seizure threshold?

• A. Acepromazine
• B. Chlorpromazine
• C. Diazepam
• D. Ketamine

Answer: (A)

In breeding stallions, acepromazine is avoided because of its alpha-adrenergic blocking action. By blocking alpha-1 receptors, it reduces sympathetic tone and relaxes smooth muscle, including the retractor penis muscle. This can allow the penis to protrude and remain prolapsed, a problem specific to stallions. At the same time, acepromazine lowers the seizure threshold, increasing the risk of seizures in susceptible animals. The other drugs don’t carry both risks: diazepam tends to raise seizure threshold and isn’t associated with penile prolapse; ketamine isn’t known for causing penile prolapse and doesn’t reliably lower seizure threshold; chlorpromazine is another phenothiazine but the combination of penile prolapse risk and lowered seizure threshold is most classically linked to acepromazine.