For regular blood donation nothing is returned to the body. That's only for things like donating plasma, platelets, or double RBCs.
I've literally never heard of anyone being disqualified for this reason.

And do you have a source on the filtering thing or is that just a guess? If the filtering process removed meds from the blood it would also be removing a million other small molecules that are necessary. And the cells in the blood would probably die because it would seriously mess up the pH and osmolarity, and free floating cells suspended in liquid are especially sensitive to such things, compared to like your skin cells which are a more solid mass.