with Cody the shots lasted a while...a few weeks at least. He had seasonal allergies though, so i couldn't be sure how long the shots lasted vs. if his allergies were over. He also had asthma, which we gave him benedryl.
when Milo was a puppy he had allergies but i just gave him benedryl for it, he grew out of it, thankfully.
If Beezey's just started with the biscuits it might be them. How long have you stopped feeding him those?