I'm pretty sure the medication in the Purina medicated is Amprolium. If it is something other than Amprolium, what I am about to say is wrong. But I do not think I am wrong.
Amprolium is not an antibiotic. It inhibits the protozoa that causes Coccidiosis from reproducing. It does not totally stop the protozoa from reproducing, but it slows it down. It is OK for some of the protozoa to be in the chick's system. That way, the chick will develop an immunity. The danger is when the number of protozoa gets out of hand. The protozoa can also live in wet chicken manure, so the real danger from Cocci is when the chickens are in a wet brooder, coop, or run. Usually, if the brooder, coop, and run are pretty dry, Cocci is not a threat, but it does absolutely no harm to feed the medicated feed to the chicks.
The medicated feed is a total waste and will do you absolutely no good if the protozoa is not present to start with. How can it inhibit the protozoa from reproducing if it is not there to start with? But you don't really know if it is there or not, so again, it does no harm to feed it.
A chick will develop an immunity to the protozoa within two to three weeks once the protozoa is present, as long as it stays active in their system the entire time. The younger the chick is, the easier it is for it to develop that immunity. An older chick or chicken is more likely to have problems than a really young chick. As long as the coop and run are pretty dry, it is usually not a big problem, but Cocci can hit older chickens pretty hard. It can hit the babies too, but older chickens are more at risk.
Your biggest risk is when they are first introduced to the protozoa. If you have taken them out to the ground or brought something in to them, they may have been exposed, but many brooder raised chicks get their first exposure when they first hit the ground. So I suggest you continue them on the medicated feed for at least three weeks after they first hit the ground. Then, when that bag of feed runs out, you can switch them to unmedicated feed like grower or flock raiser if you wish. It really does not matter if they are 6 weeks old or 12 weeks old when you switch, or even older.
I personally do not feed medicated feed and I introduce dirt from the run to them about their 3rd day so they get the exposure early. But I'll say it again. It does absolutely no harm to feed them the medicated feed and the greatest risk is when they first hit the ground.
Good luck!
