My brother used to work for one of the local pest control companies.
According to him, the reason boric acid will kill bugs, but not people or
animals is that it gives the insects gas.  They have no way to expell the
gas, and the pressure build up kills them.  Since just about all animals
(except for bugs) can pass gas, it's more or less harmless to us.  I say
more or less, because it could still cause an upset stomach, but I doubt
it would cause anything worse in an otherwise healthy animal.  Notice I
said "otherwise healthy".  I use it all the time, and so far no problems.
I've had ferrets for almost 6 years now.  I would be a little hesitant to
use it around a very sick animal.  Ferrets are so picky though when it
comes to food that I doubt they would try to eat it anyway.
 
I know boric acid used to be used a lot to clean contact lenses.  If
it's okay for your eyes, how dangerous could it be?  Always best to use
caution when in doubt, but as you can tell, I lean toward the mostly
harmless school of thought.
 
Holly,
Tigger, Pooh & Sprite
[Posted in FML issue 4261]