I'm in the US and have not heard of Reactine, can you tell me what the active ingredient is?
Also I have heard that in the UK the active ingredient in Benadryl is NOT the same as in the US (in the US it's diphenhydramine), and Benadryl in the UK is not safe for dogs.
I've just looked it up and it seems that in the UK the active ingredient in Benadryl is Cetirizine Hydrochloride, which is the same active ingredient in Reactine, so I would not give either one of these to your dog without checking the active ingredients first to make sure that it is something safe for dogs.
There is a list of SAFE antihistamines for dogs at the bottom of this page (though you will have to look up the dosage):
Loratadine (brand name is Claritin in the US) is also safe for dogs, but I'm not sure of the dosage.
NO! Your dog needs a veterinary allergy med. Never give a dog a human medication, or an animal medication, unless it has been prescribed for him. You could kill him if you do.
the only med that I know you can give an animal is baby aspirin. Do not medicate your dog with any people meds. Could prove to be fatal. Please call someone!
Benadryl is the only anti-allergy I've ever heard a vet prescribe, and even then you'll need to consult a vet for dosage, which varies depending on the breed of dog, weight, etc. Remember these allergy medications are made for people, dogs have very different immune systems and reactions to substances.
You should call your vet.
It is not an unnecessary visit. Never medicate your dog without consulting a vet. That is so dangerous.
I don't recommend it. The dosage will be far too high and it's formulated for adult humans, not animals.