A great resource for breast feeding mothers is LactMed.
The Drugs and Lactation Database (LactMed) is “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.” available at www.toxnet.nlm.nih.gov.
The Centers for Disease Control has breastfeeding information at www.cdc.gov/breastfeeding.
For those looking for in depth knowledge of every aspect of breast feeding see the Academy of Breastfeeding Medicine website available at www.bfmed.org.
All three of these sites are worth visiting.