Injections are not the way to go (for many reasons).
Mayan_Dvash -Your case is the exception rather than the rule.
Obviously the practioner felt- better to give a shot than not to give the med at all.
Believe it or not, there are protocols on which antibiotics to give for what. It’s not a mix and match game. There are books written on this. It’s a big part of medicine.
I would say get the chewables, but they too probably have the additives you don’t want.
What you do is get something called a pill crusher, crush an adult pill, it obviously has to be the same strength and then add it to some spoonful of applesauce or something similar.