Mucinex-dm-gel-caps, mucinex and mucinex dm both contain the drug guaifenesin. this is an expectorant. it helps loosen mucus from your lungs so that your coughs are more productive. a productive cough brings up the .... Mucinex dm maximum strength 12-hour expectorant and cough suppressant tablets have a bi-layer tablet with maximum-strength medicines that fight wet and dry coughs for up to 12 hours. the extended-release bi-layer tablet releases immediately and lasts for 12 hours. mucinex dm max strength contains 1200 mg guaifenesin & 60 mg dextromethorphan hbr.