carbon clears up your water and will remove metals and so forth from your water. so afetr meds, it is used to clear up all medication as it will absorb it.
If you do not use carbon, it is a good idea to replace it with another media, like filter floss or an extra sponge to make room for bacteria to roam.
I don't use carbon myself as it absorbs Iron, which plants need to grow properly. I found this product by seachem called Purigen that clears up water too, without harming plants.
to remove medication however, i would add the carbon for a day or two