Q&A: Can I Take Retin A?
Is Retin-A safe to use while I'm breastfeeding?
There aren't any studies on the effects of topical acne meds like Retin-A (tretinoin) on breastfed infants, but there doesn't seem to be any real issue with the drug. Your skin doesn't absorb it very well, so it isn't likely that much, if any, of the drug will make it into your milk supply.
However, do take care to keep your Retin-A (or other topical meds) from coming in contact with baby's skin since it could be harmful to her that way.