Guaifenesin-d 5 is the deuterium labeled Guaifenesin. Guaifenesin (Guaiacol glyceryl ether), a constituent of guaiac resin from the wood of Guajacum officinale Linné, is an expectorant. Guaifenesin can alleviate cough discomfortby increasing sputum volume and decreasing its viscosity, thereby promoting effective cough. Guaifenesin also has narcotic effect.