I've always used amoxicillin ,,,the stuff us humans use,,,as well as vets giving it to dogs and cats....
They way I have used it in the past is to ,,first isolate your ferret,,,then it's is water drinker,,,crush a tablet up in to a fine powder,,,then with a drop a hot,,,but not not boiling water,,,disovel it in the drinker,,,then top up with cold water,,,
Then instead of feeding hole carcass for a week or two,,,,feed cat food ,,,only give the ferret enough that it eats it in one go,,,so just a small amount,,, I then add 1/4 of crushed tablet,,,to this small meal,,,,if the ferret eats a