Excitability (especially in children) is one of the listed possible
side-effects of Benadryl.  You can talk to your vet about this.
Depending on the intensity or duration of the excitability your vet may
recommend a different dosage or prefer to use an injectable steroid prior
to vaccination.  It is not technically an "allergic" reaction, just an
unpleasant one, but any unusually excessive or extreme effect should be
discussed with your vet.
 
Debi Christy
Ferrets First Foster Home
www.ferrets1st.com
[Posted in FML issue 5129]