Medicated feed if only containing amprol is a a thamine blocker that prevents cocci protozoa from reporducing in the gut and making them sick. For medicated feed to work as intended, you MUST expose the chicks to the soils they will be living on as an adult so they have something to become immune to. The med in the feed just prevents an overload.
Cocci is a huge issue in some areas that are wet and moist most of the year, and non existent in others which are dry. The PNW has soil in which it thrives so I tend to use it. However, if you give day old chicks a dish of soil from the run, and don't keep the too "clean", as in no need to clean the brooder after each dropping, they will get a natural immunity on their own with a small risk of too many building up. Regardless if you give medicated feed as chicks, if you have a high percentage of cocci in the soil outside, they can ingest enough oocysts to make them sick even if they do not reproduce in the birds.