Mucinex DM 12-Hour Expectorant and Cough Suppressant Tablets are patented bi-layer tablets designed with two layers. The first layer is absorbed quickly by your body for immediate relief. The second layer dissolves slowly to continue relieving your cough for up to 12 hours. Mucinex DM contains 600mg of Guaifenesin to help thin and loosen mucus, & 30 mg of Dextromethorphan HBr to calm the cough control center and relieve coughing.