Cold medication while feeding - does it affect baby?

If you were to take a cold medicine considered to be safe to take while feeding (for instance Robitussin DM) would it affect the baby as well? I know it is considered "safe" for the baby, is that because it doesn't get into the milk in any significant amount OR because it does get into the milk but is okay for the baby to ingest? Obviously they don't make cold medicines for infants but if mom and baby both needed a cough supressant and expectorant (which is what Robitussin DM contains), do you think mom taking that medicine for herself might p on a little of the beneficial effects through the milk to the baby?