Can you take the Plan B one step pill while breastfeeding?

No, Definitely Not. There is not enough data to say for sure if it is safe while breastfeeding, but there are definitely serious side effects if taken while pregnant. I would definitely not use it.
Possibly. The "official" literature for Plan B states that the safety of the medication during breastfeeding is unknown for the breastfeeding infant. It is likely that some of the medication is found in breast milk so the infant will likely have ingested some during breastfeeding if Plan B is taken.
No, Definitely Not. There is not enough data to say for sure if it is safe while breastfeeding, but there are definitely serious side effects if taken while pregnant. I would definitely not use it.