This drug is amoxicillin on steroids. I am guessing the vet is prescribing it as a final assault attempt on all of this hen's infections, meaning there is no next thing to try if this fails. The only danger of this particular drug is that it might not work and instead render the patient immune to further effectiveness of amoxicillin, which is moot in this case as this hen is in her final days anyway.
If it's going to work, you should see improvement by 48 hours. If she doesn't, then that's it, you've then tried everything there is.
If it's going to work, you should see improvement by 48 hours. If she doesn't, then that's it, you've then tried everything there is.