Giving ducks injections in the breast is tricky. I usually just place them standing on my lap, insert needle and have DH confirm there is no blood when I draw back. If no blood, I inject. Remember that almost all birds are injected in the upper 2/3 of their breast and never anywhere else...
Welcome to BYC!
Here is the info I have for injectable penicillin:
Penicillin G
Here is some info:
Picture below is from this site: http://smartools.pharmdev.org/iu.html
-Kathy