Ferrets do not get colds.  Colds are due to rhinoviruses, which are species
specific virus.  Ferrets may get influenza, but not colds.
 
Ferrets with upper respiratory infections should see a vet prior to
starting on any medication.  Benedryl, as you say, is an antihistamine,
and generally has little effect on symptoms of upper respiratory
infection - it is really an anti-allergy medication.  Recently on this
list, we all witnessed the death of a ferret from using an over-the-counter
cold medication, based on the errant information that ferrets get colds.
Tonight I have seen another post about a ferret with a cold, incorrect
information that continues to be passed unwittingly, even on this list.
 
Flus, or mild upper respiratory infections are not life-threatening in
ferrets.  As owners, we tend to want to treat these conditions, because we
feel sorry for the animals, but over-the-counter meds are not formulated
for ferrets, and they contain ingredients that are of no benefit to
ferrets, and may even be hazardous.  For this reason, and the recent
reinforcement of the tragic death of a ferret following use of a human
cold product, that I no longer recommend these products for use in a
ferret.
 
As long as your ferret continues to eat and drink sufficiently, no
medication is generally the best approach, and if the signs appear
significant, then a vet visit is generally in order.
 
With kindest regards,
Bruce Williams, dVM
 
 -------
 
XXXX - I would direct you to the post immediately preceding this one, as
my comments on "colds" in ferrets are as valid now as they were when I
wrote them 2 minutes ago.  Ferrets that are eating, drinking, and playing
normally in spite of a mild respiratory tract infection are not candidates
for treatment, with human meds or otherwise.  Chlor-Trimeton is not just
an antihistamine, but the pseudoephedrine is a stimulant, and is not
indicated for use in ferrets.
 
The best way to help them is to give them lots of TLC, make sure that they
continue to eat, and not worry about their sneezing.
 
With kindest regards,
Bruce Williams, DVM
[Posted in FML issue 3432]