A drug that has a history of use by nursing mothers is a better choice than a new, possibly untested drug. The duration of the drug therapy can affect its compatibility with breastfeeding. A drug considered compatible with breastfeeding when taken for a few days may not be compatible when taken over a long period of time. A drug given by injection or by mouth is less concentrated than one given intravenously. However, a drug may be given intravenously because it is inactivated or not absorbed by the digestive system, so the baby's digestive system would also inactivate or not absorb the drug.
Some drugs accumulate in a baby's system and can potentially build to toxic levels.
A drug that is quickly eliminated by the baby is more compatible with breastfeeding. A number of resources on drugs and breastfeeding have been published. Not every drug available in a particular classification is listed, nor is the dosage or frequency. Drug reference books compiled by drug companies need to be used with caution. They are influenced by legal liability and the information is usually overly cautious.