To      : Dawn
Re      : Half-life

First, phenobarbital is not necessarily contraindicated in breastfeeding
infants.  From experience I can tell you that the plasma level of the infant
will probably be about to 1/4 of the mom's plasma level.  This is generally
too small to be clinically relevant.  But again it depends on the infants
stability at the time of exposure.  If you are having problems with the
health care provider, you can always suggest they monitor the plasma level
of phenobarbital in the infant.  This often reduces the physicians worries,
particularly when they find out the levels are so low.

As for the 5 half-life rule, this means that after 5 half-lives, that about
97% of the drug has been eliminated.  This is generally used to determine
when the drug is largely gone or eliminated from the respective compartment.
One half-life = 50% gone, two = 75% gone, etc to 5 half-lives = 97$ gone.

Regards

Tom Hale, Ph.D.