A top email query I receive from nursing moms is whether xxx medicine prescribed to them is compatible with breastfeeding. Since I'm no doctor, I usually tell them to ask their pediatricians but they come back saying that the doctor immediately told them that xxx medicine cannot be taken while breastfeeding so they have to stop.
The internet is a wonderful resource and I have found a lot of answers raised by breastfeeding moms there. Foremost is Dr. Sears's summary of the various lactation risks of medicines. I've found that most of the medicines prescribed are actually under the "L2" category. I also usually check Kellymom's list of medications which have been approved by the American Academy of Pediatricians as compatible with breastfeeding.
My top favorite resource is this searchable database from the United States National Library of Medicines called LactMed (Drugs and Lactation Database) which 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 level of drugs, possible effects on breastfed infants and on lactation, and alternate drugs to consider." This is my secret weapon and when moms ask me if xxx medicine is compatible, I just type in the brand or generic name and voila, I immediately get answers to the all-important question of compatibility.
Meanwhile, if you do love your pediatrician and just find him/her lacking in updates on breastfeeding and medicine, I would highly suggest that you get him/her a copy of Dr. Thomas W. Hale's 2010 Medications and Mothers' Milk - which comes in book and pocket versions. He also has an online version for registered users. Dr. Hale is recognized as an expert in lactational pharmacology (transfer of medicines and other substances into mother's milk).
So the next time your doctor says that you can't breastfeeding while taking xxx medicine, be proactive! I suggest you click on one of the resources above, search the medicine, print-out the results and talk to your doctor again. You both might just be able to come up with a happy compromise.