Guaifenesin, how much is an Overdose? My daughter 11 years old 100 lbs (big for her age), took (the correct dose) 5mL cough and cold medicine that had the following in that dose. Guaifenesin, USP 100 mg, Dextromethorpan HBr, USP 10 mL. Phenylephedrine HCI, USB 5mg. But then she also took (the correct dose) 10 mg of a prescription medicine Guaifenesin-Codeine syrup (generic for Robitussin AC syrup). It doesn't state on the label the quantity of Guaifenesin mg in the 5mL dose. How much is too much Guaifenesin?