It has been well documented that Boxers have a sensitivity to the drug Acepromazine (Ace). In 1997 there was a veterinary warning advising that it not be used for the Boxer breed. The American Boxer Club website stated:
There is one drug commonly used in anesthetic protocols that should not be used in the Boxer. The drug is Acepromazine, a tranquilizer, which is often used as a preanesthetic agent. In the Boxer, it tends to cause a problem called first degree heart block, a potentially serious arrhythmia of the heart. It also causes a profound hypotension (severe lowering of the blood pressure) in many Boxers that receive the drug. Recently, on the Veterinary Information Network, a computer network for practicing veterinarians, an announcement was placed in the cardiology section entitled “Acepromazine and Boxers.” This described several adverse reactions to the drug in a very short time span at a veterinary teaching hospital. All the adverse reactions were in Boxers. The reactions included collapse, respiratory arrest, and profound bradycardia (slow heart rate, less than 60 beats per minute). The announcement suggested that Acepromazine should not be used in dogs of the Boxer breed because of a breed related sensitivity to the drug.
Even though there have been warnings, Ace is still one of the most commonly prescribed sedatives in veterinary medicine. Make sure you know your veterinarian(s), ask questions, and air on the side of caution. Ace is never safe for Boxers and should not be used.
The Boxer World website also has a comprehensive article that you can find here and states:
There are 3 main reactions:
(1) Some vets will look aghast at the
mere mention of Ace and boxers and tell you they would never use that drug on a
brachycephalic breed (and often just don’t use it at all);
(2) Some will
willingly agree to use an alternative, even though they think you’re going
overboard about the risks; and
(3) Some will argue that it is myth that
there’s a problem and it’s all just a matter of doseage (they, of course, use a
very minimal dose), and will not or are extremely reluctant to use an