When medications are advised to take with food, it's not always to avoid stomach upset. 

Many substances cannot absorb into the body without piggy-backing on food. 

The molecules of the medicine (or vitamin, or painkiller) must bond to the molecules of food so that when the food molecule passes into the cell, the medicine, etc goes right along with it.

That means you won't even get all of our medication if you take it on an empty stomach. 

Conversely, and I cannot stress this enough, charcoal bonds with medication molecules and prevents them from being absorbed. This is why it is used to prevent or reverse overdoses of orally ingested medications. It bonds, and comes out of your body with the medication still bonded to it.

Additionally, there is no shown benefit to ingesting charcoal beyond overdose prevention. The only safe use is in a medical setting.