Horehound contains a number of constituents, including alkaloids, flavonoids, diterpenes (e.g., marrubiin), and trace amount of volatile oils.² The major active constituent in horehound is marrubiin, which is thought to be responsible for the expectorant (promotion of coughing up of mucus) action of the herb. In addition, marrubiin contributes to the bitter taste of horehound, an action that increases the flow of saliva and gastric juice, thereby stimulating the appetite.³ These actions likely explain the long-standing use of horehound as a cough suppressant and expectorant as well as a bitter digestive tonic.