Azaperone (Stresnil, Fluoperidol) is a pyridinylpiperazine and butyrophenone neuroleptic drug with sedative and antiemetic effects. It is mainly as a tranquilizer in veterinary medicine. Azaperone is officially indicated for the “control of aggressiveness when mixing or regrouping weanling or feeder pigs weighing up to 36.4 kg”. It is also used clinically as a general tranquilizer for swine, in particular with aggressive sows to allow piglets to be accepted, and as a preoperative agent prior to general anesthesia or cesarian section with local anesthesia. Azaperone has also been used as a neuroleptic in horses, but some horses develop adverse reactions (sweating, muscle tremors, panic reaction, CNS excitement) and IV administration has resulted in significant arterial hypotension in the horse; because of these effects, most clinicians avoid the use of this drug in equines. Azaperone appears to have minimal effects on respiration and may inhibit some of the respiratory depressant actions of general anesthetics. A slight reduction of arterial blood pressure has been measured in pigs after IM injections of azaperone, which is apparently due to slight alpha-adrenergic blockade. Azaperone has been demonstrated to prevent the development of halothane-induced malignant hyperthermia in susceptible pigs. Preliminary studies have suggested that the effects of butyrephenones may be antagonized by 4-aminopyridine. Azaperone acts primarily as a dopamine antagonist but also has some antihistaminic and anticholinergic properties as seen with similar drugs such as haloperidol. Azaperone may cause hypotension and while it has minimal effects on respiration in pigs, high doses in humans can cause respiratory depression which may be why it is rarely used in humans. Higher doses are used for anesthesia in combination with other drugs such as xylazine, tiletamine and zolazepam. Azaperone is also used in combination with strong narcotics such as etorphine or carfentanil for tranquilizing large animals such as elephants.