Yes, it is acceptable to use an antihistamine while on atopica. Your dog would take 50 mg of benadryl every 8 hours. Also, if you need to you can give a steroid tablet once to help with the itch - or it may be that you'll have to give the atopica every 2nd day if it is a season that your dog is exceptionally itchy.
Let me know if you have more questions,
Dr. Debbie
Veterinarian
Full time practicing companion animal veterinarian.