Medicated chick feed ordinarily does not have an anfibiotic, but rather amprolium, which affect any cocci the chicken eats by interfering with thiamine uptake in the cocci. You do have to chick labels, though, to make sure the medication is amprolium and not an antibiotic. It's been a while since I've seen feed with an actual antibiotic in it, but it does happen. TSC used to hang a sign by their unmedicated feed recommending giving them Duramycin at the same time, which is an antibiotic. I don't know whether they still do this.