Just saw this so thought I'd chime in.
It is REALLY important for anyone trying Benadryl for the first time, to try it at home first. I learned that the hard way. Spike does not like the car so on a long road trip I gave him Benadryl. As it turns out, Benadryl affects Spike as speed!
It was a horrendous 8 hours of him completely freaking out. My vet also prescribed Valium for him for his severe storm phobia and it was the same but worse. We had to lock him in his stroller to keep him from hurting himself. He was completely wired with zero coordination. It was pretty scary and I was on the phone with my vet on and off for several hours while we got through it.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226