No meds are really approved for egglaying hens, if you get right down to it. They don't want you to ever eat the eggs since the medication has not been approved by the FDA or whoever is responsible. We usually use a two to four week withdrawal for most medications, including wormers, however. Almost every medication that is used in backyard flocks is an off-label use. I don't know what Terra Vet is, though. Why are you giving it?