Good reminder to always read the label!
There are several different medications that can be found in chicken food, and the instructions are different for different ones.
I notice that you are in Scotland.
I am in the USA, where the most common medication in chick starter is Amprolium, which is considered to be safe for laying hens and for meat birds (according to the FDA which regulates such things.)
Of course that is at the present time: if anyone reads this in a few years, there might be a different "most common" medication, or the FDA might be saying something different about this one. And there are some feeds with other medications available here too, just not quite as common.
For the poster you quoted, I don't see a location in their profile, so I do not know whether your experience or mine is likely to be more like theirs, or whether theirs may be different yet.