Is an antihistamine shot for dogs a one time thing?

i have never heard of antihistamines being injected ... are you sure it was not a steroid injection ??? if it was you paid for the wrong thing, the skin scraping would have been better than a steroid shot ... and funny, i have never had a gun put to my head at the vet to make me buy anything ... and yes, that shampoo will help your dog feel better ... i have a dog with severe allergies and i had to tell vets to not give the cortisone shot any more ... it was a waste of money and if you yourself have ever had a cortisone shot, it hurts like hell to receive ... you would have been better off asking for an antihistamine you can use like benadryl ... i think it is more likely your dog has allergies and that will be a lifelong management, there is no cure ... you need to figure out on your own and it is not easy, what your dog is allergic to ... i doubt it is fleas as you have the dog on prevention and you have not found any fleas ... if you live where there are four distinct seasons and the dog is itchy year round he could be allergic to pollens and things in the environment ... in that case the dog would live on antihistamines and you would have to change antihistamine brands to keep up the efficacy ... with environmental allergies, weekly bathing with a shampoo like you bought is good ... and you can also wipe the dog with a damp cloth on coming in from outside, the coat and the paws ... if the dog is allergic to pollens and comes inside and licks those pollens, his allergy symptoms will be triggered ... you can also go onto allergy websites and see what foods crossreact with the pollens at that time of year and avoid giving your dog those human foods ... if the dog is allergic to food then you will have to do an elimination diet to find out what the dog it allergic too ... an elimination diet is for 3-4 months on one food with limited ingredients and a protein source your dog has never had before ... no other food, treats, human food, anything during this time otherwise it won't be a true elimination diet ... several companies make limited ingredient diets ... natures balance is the one i use because it comes in kibble, canned and treats (harsh to not give your dog a treat for four months :O) ... good luck, allergies suck ...