You'll get so much different information that you won't know who to believe. I'll just add to the confusion a little.
First you need to know what the "medicated" is in the medicated feed. It should be on the label. Usually it is Amprolium, Amprol, some such product, but until you read the label, you really don't know. I'll assume it is an Amprolium product, but if it is not, then realize everything I say about it may not apply. And it is possible that the "medicated" is Amprolium AND something else.
Amprol is not an antibiotic. It does not kill anything. It inhibits the protazoa that causes coccidiosis (often called cocci on this forum) from multiplying in the chicken's system. It does not prevent the protazoa from multiplying; it just slows that multiplication down. There are several different strains of protazoa that can cause cocci, some more severe than others. Chickens can develop an immunity to a specific strain of the protazoa, but that does not give them immunity to all protazoa that cause cocci. Little bitty tiny baby chicks can develop that immunity easier than older chickens.
It is not a big deal for the chickens intestines to contain some of the protazoa that cause cocci. The problem comes in when the number of that protazoa gets huge. The protazoa can multiply in the chickens intestines but also in wet manure. There is one strain of the protazoa that no matter how dry you keep the brooder and even if you feed medicated feed, your chicks can still get sick. But for almost all cases, if you keep the brooder dry, you will not have a problem.
To develop immunity to a specific strain, that protazoa needs to be in the chicks intestines for two or three weeks. The normal sequence is that a chick has the protazoa. It poops and some of the cysts that develop the protazoa come out in the poop. If the poop is slightly damp, those cysts develop and will then develop in the chick's intestines when the chicks eat that poop. This cycle needs go on for a few weeks so all chicks are exposed and they are exposed long enough to develop immunity. A couple of important points here. You do need to watch them to see if they are getting sick. And the key is to keep the brooder dry yet allow some of the poop to stay damp. Not soaking wet, just barely damp. Wet poop can lead to serious problems.
What sometimes happens is that people keep chicks in a brooder and feed them medicated feed while they are in the brooder. Those chicks are never exposed to the cocci protazoa that lives in the dirt in their run, so they never develop the immunity to it. Then, they are switched to nonmedicated feed and put on the ground where they are for the first time exposed to the protazoa. They do not have immunity, they do not have the protection of the medicated feed, so they get sick. Feeding medicated feed while in the brooder was a complete waste.
I do not feed medicated feed. I keep the brooder dry to not allow the protazoa to breed uncontrollably. The third day that they are in the brooder, I take a scoop of dirt from the run and feed it to them so I can introduce the protazoa and they can develop the immunity they need to the strain they need to develop an immunity to. To provide a place for that slightly damp poop, I keep a square of plywood in the dry brooder and let the poop build up on that. I don't lose chicks to cocci when they hit the ground.
There is absolutely nothing wrong with feeding medicated feed to chicks, whether the protazoa is present or not. It will not hurt them. They can still develop the immunity they need. But unless the protazoa is present, it also does no good.
Now, two warnings on the medicated feed, assuming it is an Amprolium product. Do not feed it to laying hens. You should not eat the eggs from hens that are eating this product.
If you get your chicks vaccinated for cocci, do not feed medicated feed. It can negate the vaccinations.
Now that I have added to the confusion, I'll wish you a good day and good luck.
