Okay, you mention penicillin on the subject, then you talk about depocillin. I don't know the difference between penicillin vs. depocillin. So I can only tell you my own experience:
penicillin G is what my vet use, they are safe to use if it is injected under the skin, ie., CANNOT FEED ORALLY
When I give Bernie penicillin G under the skin, he has the same problem. His diet dropped quite a bit, but slowly go back up in about 2 days.
w/ Pen G, I notice the needle they pre-measure and gave me is not big enough, so I ask them to swap me the ones w/ bigger gauge. Because w/ smaller gauge, whether apply to human or animal, the white liquid is so thick that it jams at the hole, in the end, it's useless. With the needle
hole bigger, the liquid goes in nicely.
Also, another trick is if you are giving Pen G which is from the fridge, make sure you let it warm up for 15 min. before inject inside her skin. One of the tech. taught me this trick as it's too cold to go inside the skin, the bun doesn't feel comfy.
The absolute best spot is the loose skin just behind the neck. See diagram