Have Cat Questions? Ask a Cat Vet Online.
Hello and thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
In my experience, benedryl is not very reliable as a sedative in cats that are nervous about riding in the car. Their stress usually overrides the effect if the drug and they continue to be anxious. Benedryl is very safe however and there is no harm in trying it. The dose is 1-2 mg per pound that your cat weighs so a 6 lb dog would get between 6-12 mg. This can be repeated two to three times per day. The pill that is available over the counter is typically 25 mg. so you may need to go with the children's liquid form which is typically 12.5 mg/teaspoon (or 2.5 mg/cc). Make sure that there are no other "cold-type" medications in it like decongestants. You may want to try a trial before the trip to gauge it effectiveness.
If it is not working to reduce their anxiety, your vet could prescribe other anti-anxiety medications which may be more effective like gabapentin or fluoxitine.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.