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.