Hives on my dog....?

You can safely (in healthy dogs) give 1 milligram per pound of body weight of Benadryl to a dog every 8-12 hours. You need to make sure it is JUST PLAIN BENADRYL and nothing with pseudoephedrine in it (Like Sudafed or Benadryl Sinus, etc). The only ingredient should be diphenhydramine HCl. Watch your dog for continued and/or progressive signs of an allergic reaction, like facial swelling, difficulty swallowing or breathing, or hives that won't go away. Difficult breathing/swallowing should be considered an emergency and you would need to go to an animal ER ASAP. Your vet can give a steroid injection to help reverse the allergic reaction if the Benadryl isn't working.