Sedating dog with benedryl
Do not feed in the car or provide a litter box in the car.
If anything get a harness and start practicing with it so you can walk the cat in rest stop areas if needed. The dosage for benedryl for cats is minute compared to what you'd get in a pill or liquid form.
The maximum Benadryl dose for a cat is 1 mg/per pound. As other people have suggested you may want to give your veterinarian a call to make sure Benadryl is safe to give your cat.
The vet told us to use it and told us how much to give her. They starting frothing at the mouth and jumping off the walls.
I've also given it to my dog, again with instructions from the vet. Ended up just letting them roam about the car while on a 24 hour road trip.
The first time she flew, the pretty red pills from the vet did nothing. She screamed for half of a 3.5 hour flight before being allowed out of her crate. (Took almost 6 hours and another dose for the benadryl to kick in.) 2nd flight only had an hour of screaming and she slept through the final flight. They were stressed for about an hour then finally settled into their spots.
She just hated the hotels but still ate drank and used her litter box.
What I recommend talk to your vet, get a crate where the cat can stand up, turn around and lay down, no more room.
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Overdoses of Benadryl are more often caused by your animal getting a box of the medication from the garbage or medicine cabinet than by being given too much.