The first thing I'd suggest is that you administer the prednisolone
every 8 hours instead of every 12 hours. My experience has been that
this makes a significant difference, and I've read posts from others
on the Ferret Health List that say the same thing.

This link is to a Tufts Open Course on ferret medicine. It's a
FANTASTIC resource that I believe every ferret owner should save and
consult regularly for advice and guidelines on dealing with just about
every ferret ailment.

http://ocw.tufts.edu/Content/60/lecturenotes/807949

Based on what you've said, you are currently giving just under 2 mg
daily of prednisolone. This is still on the low side. Most of what
I've read, including past posts on the Ferret Health List from vets,
suggests that up to 4 mg daily per kg is okay. Of course, desperate
times can call for desperate measures, and if a ferret is in late
stages of insulinoma or lymphoma, then giving a higher dose than that
is preferable to seizures or death, but there does come a point at
which you will notice that giving more pred just doesn't help any more.

You might also consider adding diazoxide to help lower insulin levels
in the blood. It works for some ferrets, but doesn't help much for
others.

Another possibility in later stages of insulinoma is a stronger steroid
like dexamethasone.

The highest I've gone with a daily dosage is 4 mg, but by that point my
girl Minnie Mouse had highly aggressive cancers, and even that dosage
didn't help very much.

For anyone who's read this far, I also have one other recommendation to
save a lot of money. I had been purchasing prednisolone (15 mg/5 ml)
from a local pharmacy up until mid-year of 2010. At that point I tried
a new source. Pet Health Pharmacy in Arizona:

http://www.pethealthpharmacy.com/

They will compound any strength prednisolone, and it's considerably
cheaper than most other sources. For example, I got 30 ml of 10 mg/ml
prednisolone, shelf stable, and on a per-mg cost basis, it ended up
being 1/6 of the cost of what I had been paying locally. And it
certainly worked well. I also got diazoxide there at a fraction of the
cost that I had previously paid elsewhere, and they also compounded
that to a stronger strength for me.

Jeff In Loving Memory of Neo, Trinity, Morphy, Possum, Pip, Pop,
Sabrina, Minnie Mouse, Misty, Frodo, Baggins, and Mr. Parker
Caring for Luna, Baby Girl, Dozer, and Hunny

[Posted in FML 6931]