Terpin is a crystalline saturated terpenoid glycol that is obtained from sources such as turpentine, oregano, thyme, and eucalyptus. It is commonly used as an expectorant to loosen mucus in patients with bronchitis and related conditions. Terpin hydrate is typically formulated with an antitussive such as codeine as a combined preparation. Terpin hydrate works directly on the bronchial secretory cells in the lower respiratory tract to liquify and facilitate the elimination of bronchial secretions. It also exerts a weak antiseptic effect on the pulmonary parenchyma. Terpin hydrate was used in the United States in the late nineteenth century, but was removed from marketed medications in the 1990s after the Food and Drug Administration (FDA) found a lack of evidence of safety and effectiveness. Elixirs of terpin hydrate are still available with a prescription, but must be prepared by a compounding pharmacy.