This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Common horehound, or Marrubium vulgare, is a perennial herbaceous plant, a native of Europe, but introduced into the United States, where it grows abundantly along the roadsides. The whole herbaceous part is used. This has, when fresh, a strong, rather agreeable odour, which is diminished by drying, and lost by long keeping. The taste is bitter and lasting. The herb yields its sensible properties and medical virtues to water and alcohol. These depend on a bitter constituent and a volatile oil. It contains also a little tannic acid.
Horehound has been known as a medicine from the times of Charlemagne. In reference to its effects on the system, it is mildly tonic and gently stimulant, and is thought also to be somewhat diaphoretic, diuretic, and laxative. If we may judge of the opinion entertained of its action by the use made of it, we must add to the properties just mentioned those also of an expectorant. The complaints in which it has been given are dyspepsia, chronic hepatitis, jaundice, amenorrhoea, various cachectic affections, chronic bronchitis, pulmonary consumption, and ordinary catarrh. What good can be effected, in these and other complaints, from a mild tonic, which, when taken in warm infusion, may gently promote the cutaneous and bronchial secretions, and perhaps the uterine, may be expected from horehound, but nothing more. It is at present seldom used by regular practitioners, and, even as a domestic medicine, is chiefly employed in catarrhal affections of the air-passages. The dose of the powder is from thirty grains to a drachm. The infusion is made in the proportion of an ounce to a pint of hot water, and given in wineglassful doses. A syrup is often prepared from it, and a candy impregnated with its taste is sold in the shops; both being used for ordinary colds.