Mucinex DM Maximum Strength: Take 1 tablet by mouth every 12 hours. No more than 2 tablets in 24 hours. What drugs interact with guaifenesin and dextromethorphan? Guaifenesin/dextromethorphan should not be used with MAO inhibitors like phenelzine (Nardil),selegiline(Zelapar, Emsam, andEldepryl), t...