I also have had a problem with a ferret acting very tired.  In my case the
vet says it is probably low blood sugar (this is not an uncommon problem
with older ferrets).  She advised me to give him a little honey or Karo
syrup several times a day to help regulate his blood sugar level.  I have
been doing it and this has helped a lot.  I'm not a vet but could the
antibiotics be affecting Elbert's blood sugar?  You might try giving him a
little Karo and seeing if it will help.
[Posted in FML issue 1911]