Before you can talk about "medicated" feed, you need to know what the medicine is that makes it medicated. Some are related to coccodiosis only and have nothing to do with anything else. These are even split into types that allow the chicks to develop and immunity and those that don't allow them to develop immunity. There are some that may work on other things than coccidiosis. There are some that handle coccidiosis plus other things. When I go to the medicine cabinet to get some medicine, I look to see what kind of medicine it is before I take it. You should be able to get some pretty good information by reading what is on the bag, but you might need to do a little more research to know what you are dealing with. The words coccidiostat and coccidiocide are good clues. Coccidiostats allow immunity to develop. Coccidiocides do not allow immunity to develop.
With all that said, a majority of chick feed sold as medicated contains an Amprolium-like product in a concentration that, if that is all they eat, will help reduce the chances of them getting coccidiosis and will allow them to develop immunity. It is a coccidiostat in that concentration. Coccidiosis is caused by protozoa that can live in the ground. There are different kinds of this protozoa. Immunity to one does not give immunity to all. This protozoa can live and reproduce in the chickens intestines. Normally some of this protozoa is not a problem. The problem is when the protozoa multiplies to such great numbers it can cause a problem. This protozoa can live and multiply in wet chicken manure. Most of the time wet manure contributes to the chicks developing a problem with coccidiosis. The chicks eat the wet manure and the numbers get out of hand. A fairly dry brooder is a real good safeguard against problems with coccidiosis.
The way the Amprolium works, it interferes with the protozoa multiplying in the chicks intestines, thus keeping the number down. It does not stop all reproduction, just stops some reproduction. The chick can still have a problem with coccidiosis if the brooder is wet.
The Amprolium will do absolutely no good if the protozoa is not present to start with. It won’t hurt anything, it just won’t help. If your chicks are raised in a brooder and never see dirt, they are unlikely to be exposed to the protozoa until they leave the brooder and are exposed to dirt. Depending in how you raise your chicks, it may make more sense to feed them medicated feed when they leave the brooder for two or three weeks.
I purposely put some dirt from the run into the brooder about Day 3 so they are exposed to any protozoa while they are young when they can best develop immunity. I do not feed medicated feed but keep my brooder fairly dry. I’ve never had a problem with coccidiosis. If I had, I might do differently.
Amprolium in dosages higher than what is normally in medicated feed is used to treat coccidiosis. The dosage in medicated feed is not that high. It should be used as a preventative and not a treatment. Coccidiosis can kill chickens. If you have an active case, you need to actively treat it.
If you feed things other than just the medicated feed, you reduce the dosage levels accordingly. It will still inhibit the protozoa from reproducing, just not inhibit it as much. There is nothing wrong with that. It will still help, if the protozoa is present to start with.
A lot of the research done on this and a lot of the recommendations for these are made for commercial flocks. Most of us are not raising the thousands of chicks the commercial places raise and our conditions are normally different. I read some of those reports and studies but I try to convert them to my situation, which is not a mass commercial situation.
Hopefully you will get something from all this rambling to help you. Good luck!!