N-acetyl-cysteine (NAC) is derived from a naturally occurring amino acid, cysteine. NAC breaks down mucoproteins, thereby reducing the viscosity (thickness) of bronchial and lung secretions. Bromelain is an enzyme from the pineapple plant (Ananas comusus) that breaks down proteins, thereby reducing the viscosity of mucus. It also helps clear the airways and suppress coughs.
Marshmallow (Althaea officinalis) root extract contains a polysaccharide, rhamnogalacturonan, which inhibits the cough reflex and modulates airway reactivity. Mullein (Verbascum thapsus) contains mucins that reduce inflammations of the upper respiratory tract and saponins that dissolve phlegm and encourage expectoration. Wild cherry tree (Prunus serotina) bark contains organic acids, which help loosen and clear mucus from the lungs.
Horehound (Marrubium vulgare) contains a medicinally active compound called marrubiin that exerts a direct stimulatory effect on the secretions of the bronchial mucosa. Ivy (Hedera helix L.) leaf extract reduces asthma attacks and improves COPD by reducing bronchial spasms and improving respiratory secretions.
Herbal expectorants which expel mucus (such as extracts of wild cherry bark, mullein, and horehound) work best when combined with mucolytics which break down mucus (such as bromelain and NAC), and with demulcents which soothe irritated mucous membranes (such as extracts of marshmallow and mullein).