Some similarities to: Motherwort but the leaves of Motherwort are very different, having three lobes.

Not to be semantically confused with : Houndstongue [a plant with similar name]

Not to be confused with the closely related species: White Horehound (Murrubium vulgare) which is thyme scented and is so smothered in white hairs as to appear grey-green.

Many similarities to Hedge Woundwort but that has deeper purple flowers with white markings on the lower lip, which is also not lobed as conspicuously as is Black Horehound. Also, Hedge Woundwort has no sterile sepal tubes; all will have flowers in them. Black Horehound has many more flowers in the whorl. Both smell pretty dire.

Black Horehound is renown for its highly disagreeable pungent smell when crushed, but some folk actually like this aroma. So far, the author has been unsuccessful at finding the chemical compound(s) responsible for this foetid smell.

DITERPENOID LACTONES of the LABDANE TYPE Black Horehound contains half a dozen Diterpenoid lactones of the Labdane type, compare Larixol. Most have an additional Furan ring (top right), or two in the case of Preleosibirin. All are loosely based on Marrubiin. Marrubiin is also found in White Horehound (Marrubium vulgare), a related species. One reference quotes Black Horehound as not possessing Ballonigrin (despite the name) and instead says it is present in several other Ballota species instead, but not Ballota nigra. Preleosibirin is a Prefuranoid with, unlike the above, two acetyl groups (lower left) and two five-membered rings (upper right). It can easily be envisaged that the carbon chain linking the six-membered rings to the five-membered ring of the other five comes about as a result of the breaking of the oxygen bridge bond on the first five-membered ring (TetraHydroFuran, but the author cannot be sure.

FLAVONOIDS Black Horehound also contains the flavonoid Luteolin-7-O-Lactate and its glycopyranoside as well as a poly-methoxylated flavone called Tangeritin, which is present in the peels of Tangerines and other citrus fruits. Tangeritin strengthens the cell walls and also has a cholesterol-lowering effect in the human body, and may also be useful against Parkinson's Disease. It will selectively induce apoptosis in Leukaemic cancer cells whilst ignoring normal cells. The flavonoids showed some anti-fungal activity against Candida species.

PHENYL PROPANOIDS and their GLYCOSIDES These glycosides are all based upon the non-glycoside (+)-(E)-Caffeoyl-(L)-Malic Acid, which is also present in Black Horehound. Black Horehound also contains the following PhenylPropanoid Glycosides within the flowers: Verbascoside, Arenarioside, Forsythoside B, Ballotetroside, Alyssonoside, Lavandulifolioside and Angoroside A. These compounds are responsible for the neurosedative activity of Back Horehound, which prolongs sleep. They are able to bind to dopaminergic, benzodiazepine and morphinic receptors. The two glycosidic units present in Verbascoside and all the other Phenylpropanoid Glycosides shown here are glucopyranose and mannopyranose. Some of these compounds demonstrated anti-fungal activity, with Arenioside being effective against methicillin-resistant staphylococcus (MRSA). The first one, Verascoside, has but two glycosidic units, the next five have three glycosidic units, whilst the last, Balletetroside, has four.

Pinene and Humulene have also been found in the volatile oil from Black Horehound.