– Terpin hydrate, a cis-form hydrate, is derived from sources such as oil of turpentine, oregano, thyme, and eucalyptus. It is used as an insect repellent and expectorant. Terpin hydrate(cis-Terpin hydrate) is commonly used to loosen mucus and ease congestion in patients presenting with acute or chronic bronchitis, and related pulmonary conditions. It was popular in the United States since the late nineteenth century, but was removed from marketed medications in the 1990s after the U.S. Food and Drug Administration (FDA) stated that “based on the evidence currently available, there are inadequate data to establish general recognition of the safety and effectiveness of these ingredients”. Elixirs of terpin hydrate are still available with a prescription but must be prepared by a compounding pharmacy.