Should You Be Taking Mucinex and Breastfeeding? Taking Mucinex while breastfeeding is considered fairly safe, but there have been no conclusive tests. According to the American Academy of Pediatrics, the active ingredient in Mucinex, guaifenesin, is considered to be in the L2 (safer) category. Basically, they don’t know of any ill effects but there have not been any controlled studies in humans or animals.