Boxers should not be given any anesthetics or sedatives that contain Acepromazine.
Need to know information:
Boxers have a documented sensitivity to the drug Acepromazine (aka Acetylpromazine, common brand names are PromAce, AceProject).
Acepromazine (Ace) can cause a severe lowering of blood pressure which can lead first to respiratory and then cardiac arrest. A veterinary warning was issued in 1997, which you can view on the American Boxer Club website: http://www.americanboxerclub.org/ace_boxers.html
You need to know this because Ace is still one of the most commonly prescribed sedatives in veterinary medicine and NOT every vet accepts that it can kill boxers and should never be used with this breed. It is never safe: A dog that has had Ace previously without issue can collapse and die a later time it is given it.
When choosing a vet
ASK them about their anesthesia protocol, and use of Ace in boxers (if you need to, print out the warning linked above from the ABC to take with you). Your vet may also refer to their copy of the Handbook of Veterinary Drugs, where the Acepromazine section states "Prolonged effects of the drug may be seen in older animals. Giant breeds, as well as greyhounds, appear quite sensitive to the clinical effects of the drug, yet terrier breeds appear more resistant. Boxer dogs, on the other hand, are predisposed to hypotensive and bradycardic effects of the drug."
There are 3 main reactions to this:
(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 alternative sedative.
Ace is a life and death matter, and not one of dosage. Obviously, the first group are the vets you want - and the third the ones you should never use. In any case, you should ensure that "NO ACE" is recorded on your dog's file in large, unmissable letters.
Remember that you are the client. Your vet (or vet tech) has no business insisting upon the use of drugs if you are not comfortable with their use, and have declined to give your consent. If you encounter such a vet, take your dog elsewhere.
Acepromazine is commonly used as a pre-anesthetic sedative, therefore, if your dog is having a surgery of any kind, you need to ensure that any sedatives used will not include Ace.
You can read more about this on the boxer discussion forum.