There is a lot of debate regarding the use of medicated or un-medicated feed.  Depending on who you ask, you'll find that folks are pretty opinionated.  Amprolium is the product in question.  It is a Thiamine blocker.  The Coccidia organism needs Thiamine to replicate, so it can't if Amprolium is in the chick's gut.  Theory goes that chicks will have the benefit of medication while they build tolerance to the organism that is in all soils.  Down side is that they often are taken off the med just when they are turned out onto the soil.  The more natural approach, which I use is to expose them to my soils very early, within the first 2 weeks, (this is when they have the most antibodies in their system) so they can start building tolerance as well as getting healthy gut flora from the soil for a more natural flora balance.  I've never had a bird with coccidiosis, and do not want to block Thiamine in my chick's diet.  Broody raised chicks don't have issues with coccidiosis.  Mama takes them out right away and they start eating soil, as well as picking at chicken feces in the yard to build their gut flora.