The active component, ivermectin, is the same. The carrier molecule in the injectable solution is propylene glycol. This compound has been tested by the US FDA, and is generally listed as safe. It is used as a replacement for ethylene glycol in those "pet-safe" antifreeze solutions that you can buy. Would it be okay to give a couple of liters of propylene glycol orally? No, probably not. In this formulation, you are giving a very, very tiny amount.
As far as the sheep drench, it is an oral product. If you look at the dosing recommendations, you will see that it is a very tiny amount. If you have any doubts about this, I suggest you continue to buy the oral product that is sold through your vet. after all, it went through a 30-day test.