Very little medical research has been done about the safety of even common medications during pregnancy and breastfeeding. Mothers have therefore faced nearly a full ban on over the counter and prescription medications except in the most dire of circumstances.

During pregnancy the primary concern is whether a medication might cause birth defects or negatively affect body systems that support pregnancy. During breastfeeding we must consider both the medication's effect on the mother's milk supply and also the effect it might have on the child through her milk. Fortunately during breastfeeding there is much more flexibility to take medication.

The best person to ask about particular medications is always your doctor or pharmacist. The FDA has developed a rating system called the FDA Pregnancy Risk Categories, you can read more about them here: http://safefetus.net/fda_category.asp  Thomas Hale, a leading doctor in the field, has developed a rating system for medications during breastfeeding called the Lactation Risk Categories: http://kellymom.com/health/meds/lactation_risk_cat.html

Dr. Hale also publishes a book called "Medications and Mothers' Milk" which is the definitive resource for breastfeeding mothers. You can order the book online here: http://www.ibreastfeeding.com/shop/

Sometimes you want to be able to do your own research online before talking to your doctor. The website I found most thorough during pregnancy is http://www.safefetus.net which allows you to search for medications by both their generic and trade names. For breastfeeding mothers there is a very good reference available at http://www.kellymom.com/health/index.html