Why does the dosage of oral drugs differ to that of intravenous?

The simple answer as to why dosages of oral medication differ from intravenous (IV) doses of pharmaceuticals is that to be effective, oral medication must be absorbed in the digestive system. Anything given in an IV is in you and that means all of it. Yes, your body can process out some of the IV medication, but this is different than your body having to absorb it in the gastrointestinal tract.