Here is an article I came across that I thought you would find very interesting! I'm only going to copy out the summary since it's a bunch of medical jargon, and the summary lays it out clearly! :) Enjoy.

Adverse Drug Reactions in Breastfed Infants: Less Than Imagined

Summary: Medication use during breastfeeding shortens the duration of breastfeeding often because of overly cautious information given by healthcare providers. No comprehensive review of the literature on infant adverse reactions from drugs in breastmilk has been published. All published studies and case reports on adverse events in infants caused by medications (excluding drugs of abuse) in breastmilk were identified and analyzed. Of 100 case reports evaluated, none were considered to be "definite" using a standard ranking scale; 47% were "probable" and 53% were "possible." Drugs with central nervous system activity accounted for half of all reports. All 3 reported fatalities involved central nervous system depressants, but each had extenuating circumstances. At least 63% of reported cases were in neonates and 78% were in infants 2 months or younger; only 4% of adverse reactions ocurred in infants older than 6 months of age. Published studies expand on and generally reinforce the analysis of case reports. By taking a few simple precautions in drug selection and considering the infant's age, breastfeeding rarely needs to be discouraged or discontinued when a mother needs drug therapy.

Clinical Pediatrics. 2003;42:325-340.