White horehound (Marrubium vulgare) is an annual or perennial flowering plant in the mint family. It is native to the Mediterranean region and Asia Europe but now has a global distribution. It has a woody, branched taproot and numerous stems from 20 to 100 cm high. The use of white horehound as a folk medicine has a long history and its significance in traditional medicine is indicated by a large number of pharmacopoeias, standard text books, monographs and scientific reports which refer to it. The fresh or dried leaves and flowers of the plant are used to make herbal remedies which are typically used either in teas or syrups. Its primary use has been as an expectorant or for coughs and minor respiratory ailments but other reported uses include the treatment of digestion problems including diabetes, loss of appetite, indigestion, bloating, gas, diarrhea, constipation, liver and gallbladder complaints and difficult or irregular menstrual periods. It is characterised by the FDA as generally recognized as safe and extracts of white horehound are used as flavoring in foods, beverages and candies.
Agnieszka KwiecieĊ, Nova, CC BY-SA 4.0, via Wikimedia Commons
The principal active constituents of the plants are diterpenes. Marrubiin is one of these and is converted into marrubinic acid to which is appears to be responsible for the choleretic property of the drug. Related compounds present to a lesser extent are the diterpene alcohols marrubenol and vulgarol. Other constituents include flavonoids such as vitexin, apigenin and luteolin together with their glycosides, the alkaloids betonicine and stachydrine, and a small amount of volatile oil comprised mostly of monoterpenes which give the drug its pleasant smell.
Marrubium vulgare also has potential use as a natural pesticide and has been used in Spain on chicken farms to prevent lice. There are akso indications that extracts of the plant are effective against larvae of the mosquito Culex pipiens and other species. These extracts can be also used as a source of natural herbicides to control weeds in crop fields. On the other hand, the plant is drought-tolerant, spreads easily and can thrive in poor soils, and for this reason it is considered an invasive species in a number of countries including the USA and Australia.