Could you tell me what is the highest safe amount of robitussin (guaifenesin) to take?

Guaifenesin: The active ingredient in Robitussin and Mucinex is 'guaifenesin' which is recommended up to 2400 mg max dose per day in divided doses. However, it is an extremely safe medicine in general with few side effects and no well known toxicity. Robitussin usually contains other meds, like phenylephrine or dextromethorphan which have more toxicity. Plain guaifenesin is the safest drug in those mixtures.