Buying a few extra pills from your veterinarian, and testing your pet with a dose on a day when you're home to observe the effects would be a wise move. That way, Needham says, "if the animal has a reaction to the drug, you'll be able to quickly get him to treatment," something you can't do if the bad reaction is occurring beneath your seat at 30,000 feet, or worse, in a baggage compartment you can't see or get into.
There are times when even airline personnel appreciate that an animal is tranquilized. Former baggage handler Hiram Marquette says: "Once, we opened a luggage compartment, and there was a big boxer inside--belonged to some king. Dog got so excited when we opened the baggage compartment, he destroyed his cage, then came out at us. That was one mad boxer. Tough problem to deal with; you get into a lot of trouble for shooting a king's boxer."
The first caveat for animal air travel is: Assume nothing. You don't want to find out about some obscure-but-inviolable restriction half an hour before flight time. Ask and ask again. Also, if your flight involves connections, don't take for granted that the rules or in-flight conditions that apply to one carrier or type of airplane apply to another.