ANYTHING that really knocks down a ferret causes hind leg weakness.
 
Yes, anaphylactic reactions can cause brain damage but that is very rare
and tends to involve a situation in which a wait took place between the
reaction and treatment -- like when a person is not near medical care and
doesn't have self-carry meds along when a reaction begins.
 
More commonly, it takes time to recover.  The epinephrine provides an
artificial lift along with reducing the reaction since it is the same
thing as adrenalin, and the Benedryl causes drowsiness for a few hours,
then there is simply waiting for the body to have all the mucus membrane
swelling go down, replace the fluids lost or re-directed, replace blood
cells that were washed from the intestinal capillaries by the violent
loss of fluids that had been in them, and if there was a convulsion then
time to recover from any soft tissue injury which might have happened in
that.
 
I'm not a vet, but we've had ferrets react that badly, and way against
the odds Steve and I are both people who go into this type of reaction
ourselves for assorted things so we're kind of familiar with the
situation from multiple perspectives.
 
****IF**** he is not dehydrated then give the bit of added time unless
a dangerous symptom appears, but if the weakness remains then have a
follow-up vet visit.  He sounds dehydrated...
 
Dehydration is dangerous and it can damage kidneys, cause the kind of
problems you describe, and cause a refusal to eat due the mucus membranes
being too dry to smell food and a failure to drink due to a mixed up
sense of thirst from it.  If he may be dehydrated call the vet and get an
emergency appointment for sub-cu fluids today.  If his gums are tacky,
sometimes if his eyes lack luster, sometimes if his skin doesn't bounce
back when scruffed then get him a sub-cu promptly.
 
For feeding first calm yourself with long, deep, slow breaths since if
you seem nervous the ferret won't eat.  Mix a meat-only baby food or a/d
with warmed water or an electrolyte replacer like Pedialyte or Kayolyte
till it is about body temp so has more odor.  Give that from a spoon,
bowl, or fingers if that is needed, with praise and singing to keep
ferret happy.  Kibble can be hard on a sore gut and is often advised to
be avoided in any form during recovery from anything that irritates the
intestine.
 
Syringe feeding is a very last resort because there is a chance of
causing aspiration pneumonia, so if you have to do that then be sure that
you have equipment which has a lot of control for you (like the clown
face "spoon droppers" (which don't have spoon but are large syringes with
wide openings and an accordian pleated top with a clown face on it) and
never put in too much at once, plus allow a lot of breathing breaks after
each bit.
 
Rather than being most worried about his hind leg weakness at 48 hours
out I am much more worried by the chance that he is dehydrated.
 
Don't change any long term plans right now.
 
More info can be found by using the archives for the FML and the FHL:
http://fhl.sonic-weasel.org/ and
http://listserv.cuny.edu/archives/ferret-search.html .  Yes, you
should pretreat.
 
Ferret mention and photo:  Page 62, "Mac Addict", September issue...
[Posted in FML issue 3865]