First off: For most individuals plain pediatric Benedryl (the versions
without additives, please, since additives in some meds like Tylenol can
be fatal for ferrets) is a very forgiving medication.  You'd be shocked
at the amounts they have to give me IV when I have general anesthesia and
for a few days afterward.  The rule of thumb though, is that when a very
large dose is given then there needs to be supervision by medical
personnel at that time.
 
Pretreating is not done for sedation; it is done for the same reason
that people who are prone to the severe form of allergic reaction called
anaphylactic shock are pretreated before getting allergy shots (and, yes,
it can be worth the risk when the person's lungs and life-style are
compromised enough): to reduce the chances of a reaction and the severity
of a reaction when one occurs.  Fortunately, by now most people know to
separate the shots (not give them in the same appointment) and to stay in
the vet hospital for at least 45 minuted following a vaccination in case
an anaphylactic reaction occurs.  Because that results in rapid treatment
deaths from such reactions are extremely rare.  (This, BTW, is why your
allergist says that adults and children who can express themselves should
stay in the office for 20 minutes after shots -- longer for those who can
not clearly express themselves, and that if any of a number of symptoms
(flushing, throat closing or sore, difficulty swallowing, feeling of
impending doom, hives, swelling, tiredness, sleepiness, etc.) appear that
the nurses should be immediately notified so that the allergist can
attend to it with medications which are similar to those used for the
ferrets when they react.)
 
BTW, I checked with multiple noted research professionals on allergies
and on cardiac health and the last time I checked (sometime last year, I
think) there were no indications in many mammals that past anaphylactic
reactions increase the risk of later cardiomyopathy happening, so that
old observation was hopefully a coincidence.
 
(BTW, going form my personal experience, that of Steve and of 3 other
members of our families who go into anaphylatactic reactions except when
a feeling of impending doom occurs the symptoms look worse than they feel
unless they advance the point where a respirator is needed or unless
extreme dizziness, cramping, or over-response to the epi occur.  When
bloody runs occur during such a reaction it is not that the intestines
are shredding or such; what is happening is that the fluid is being moved
out so rapidly from the cappilaries that it is actually carrying red
blood cells along and it just feels like having the worst runs of your
life -- which is likely is unless you have had something like one of the
old scurges that are mostly under control in industrial nations -- but
no worse than that in pain so folks shouldn't imagine worse.)
 
An allergic reaction occurs when the body became convinced after a
previous exposure that a substance was an invader.  The part of the
immune system which tends to go off-kilter is one that normally handles
parasites.  Then the next time there is an exposure the reaction occurs.
This is why true food allergies tend to be to preferred foods, why most
allergy onsets happen in adulthood, etc.  There are many hypotheses right
now about why allergies happen: from homes that are too clean, to too
much exposure to certain substances in susceptible individuals(such as
favorite foods, high pollen counts, common matrices in vaccines, etc.),
to genetics (tendency to allergies tends to run in families), to
pollution levels increasing susceptibility, and so on.  An anaphlactic
reaction tends to kill by either smothering or by dipping the blood
pressure too low to remain alive.  (There are other serious types of
allergic reactions which can be fatal -- from asthma, to one in which
the tongue swells enough to shut off the airway, to a very rare form
(luckily) of dermal necrosis which is a true horror.)
[Posted in FML issue 3916]