What is "terpin hydrate"

Terpin hydrate is an expectorant, commonly used to loosen mucus in patients presenting with acute or chronic bronchitis, and related conditions. It is derived from sources such as oil of turpentine, oregano, thyme and eucalyptus. Though it was quite popular in the USA since the late nineteenth century, it was banned by the U.S. Food and Drug Administration (FDA) in the 1990s due to lack of proof of efficacy.