We give our dog Benadryl for long car trips, because of car sickness. The vet recommended our dog take 1 adult Benadryl. (She said 1 milligram for every lb. of dog.) The Benadryl makes our dog sleepy, and a bit out of it, but, he still will not sit or lay down in the car. He stands with his car seat harness on, and just stare straight ahead. After the trip, he sleeps a lot.