I have heard of this method as well for a foster pup we had that got car sick. and he was a bit larger than a yorkie so it was a disaster when we would take him anywhere
we gave it to him and he was fine on it. he got a little groggy but that's about it. never had any effects other than that and he is completely fine and healthy. My vet also said benedryl but I felt that was a little more iffy.
good luck!!!!