Guaifenesin-d3 is intended for use as an internal standard for the quantification of guaifenesin by GC- or LC-MS. Guaifenesin is an expectorant. It inhibits production of mucin 5AC (MUC5AC), reduces mucus viscosity and elasticity, and increases the mucociliary transport rate of endogenous particles in primary human tracheobronchial epithelial cells in a concentration-dependent manner. Guaifenesin increases phenol red secretion, a marker of expectorant activity, in rats.3 Formulations containing guaifenesin have been used as expectorants in the treatment of the common cold and chronic bronchitis.