See what the feed is medicated with. Usually the medicated feeds are just amprolium. If this is the case, it is just a thamine blocker that helps the chicks along in building their immunity to cocci. If it is sulmet and amprolium, it does the exact samething, just dont' feed it past 16 weeks because sulmet is a sulfer containing drug that some people are allergic to and can be passed to eggs.
I go with medicated because it helps them build their immune system up faster. It is not necessary though as not all places have cocci prevalent in the soil. I have done it both ways. Just had once case of cocci when I raised summer chicks on non medicated feed and there was a heavy rain that brought out the oocyts from hibernation and the chicks ate too much dirt with the oocysts in it.
In short.
Medicated feeds are usually juts amprolium. It is a thamine blocker, NOT an antibiotic or hormone.
Mediated feeds Prevent cocci and help Build resistance, not stop cocci infections.
If the medicated feed contains more than amprolium, do not feed past 16 weeks of age.