What are the chances of developing side effects from mucinex (guaifenesin) fast max severe congestion and cough?

Side effects: There are side effects with almost every medication you take-over the counter and prescription. Difficult to predict who will experience them and to what extent. Taking the medication as directed can decrease the likelihood in some instances but never any guarantees. Sometimes it's a matter of do the side effects make you feel worse than the medication makes you feel better. Good luck!