Will Falconer, DVM Moderator
Nope. You'll see an improvement almost immediately when you switch, except if you have an allergic dog with multiple allergies. That's why z/d is often prescribed. There are no foods left that the dog doesn't react wildly to, so they put them on protein sources that have been very manipulated to appear "novel."
If that's how he got on z/d, I'd recommend hiring a veterinary homeopath to cure the allergy state. See my Resources page for the AVH list.
Oh, and you'll want to never vaccinate him again. That's what made him sick in the first place. See Immune Confusion to learn more. And a series of vaccination pages.