Most prescription and over the counter drugs, including contraceptives, are considered safe while breastfeeding. A few medications are not safe while breastfeeding. The decision is made by assessing the risk/benefit ratio of a small amount of a drug compared to the tremendous benefit of breastfeeding to the infant.
In general, only small amounts of medications that are acidic, water soluble, highly protein bound, and with low oral bioavailability pass into milk. Drugs of large molecular size do not cross the membrane into the milk. If a mother can take a medication while pregnant, or if the medication is given to infants, it is usually compatible with breastfeeding.
Resources for information on maternal medications during breastfeeding:
United States National Library of Medicine -
Drugs and Lactation Database (LactMed)
A peer-reviewed and fully referenced database of drugs to which breastfeeding mothers may be exposed. Among the data included are maternal and infant levels of drugs, possible effects on breastfed infants and on lactation, and alternate drugs to consider.
Drug Entry into Human Milk
Breastfeeding and Human Lactation Study Center at University of Rochester
The Breastfeeding Pharmacology website of Dr. Thomas Hale: http://www.infantrisk.com/category/breastfeeding.
Berlin CM. Update: Transfer Of Drugs and Chemicals into Human Milk, Breastfeeding Abstracts, August 2001. (21), 1: 3-4: http://www.lalecheleague.org/ba/Aug01.html