I use only Ezekiel Bread as it is flourless and only sprouted grain. All I eat myself. Anything I put on it, my ducks just gobble down and it doesn't keep them from eating their regular food.
 I put portions of antibiotic powder on a small piece of it, syringe water drops on it until the powder just sinks in. It may take several small pieces for the full dose. My ducks like to eat this from my hand and suspect nothing ominous. Liquid meds work this way also. Mixing in water to drink has not been good because they never drink all of it before it gets mucky. I just provide clean water nearby. They can wash it all down. If one has many ducks this would be difficult but for my two Pekins or a small number it can work.