No such thing as a stupid question.

Drugs are handled by the body in several ways.  They can be pooped out.
The canned be peed out.  They can also be changed into something else.
Phenobarbital is partially excreted unchanged in the urine.  It is also
metabolized by liver enzymes into something else.  The something else
can also be pharmacologically active or not.  Prednisone is metabolized
to prednisolone, which is actually the active ingredient, for example.

Being on a drug often induces the liver to produce enzymes which
metabolize that drug.  Bacteria do that too when they become resistant
to antibiotics (this is just one way they become resistant).  So while
the mother takes phenobarbital during pregnancy, the baby's liver is
learning to make enzymes which metabolize it.  This would happen anyhow,
but it might result in phenobarbital accumulating if the baby was first
exposed to it only once he was born through the milk.

Jack Newman, MD, FRCPC