Compare to the active ingredient in Maximum Strength Mucinex. Say goodbye to chest congestion and bothersome. mucus with Guaifenesin Maximum Strength Extended-Release Tablets 1200 mg. This maximum strength expectorant. works for up to 12 hours to provide chest congestion relief. Just one dose of this extended-release tablet releases. the medicine you need immediately and lasts for up to 12 hours to help loosen phlegm (mucus) and thin bronchial. secretions in the airways to makes coughs more productive.