Benadryl will not hurt them. Follow the weight directions as you would for a child. They make a child's formula that melts on contact in the mouth which should bring fast relief, but you will need to give it every 4 to 6 hours. It's more important to find out WHAT caused the allergic reaction and remove it permanently. If the reaction isn't better by the second dose, then you should contact a vet. Again, find out what caused the reaction.
My Dog Has Had An Allergic Reaction, Have Given Him Piriton But Hasn't Helped. He Is Itching Like Mad. Can You HELP?
You should never give anything except benadryl. Take him to the vet to make sure that he is going to be o.k. Please. Hope this helps, good luck to you and the pup.
Try a localised anti histimine like Anthisan and apply directly to the affected area . Failing that I would see a vet x