While I was researching this article, I was surprised to discover that horehound was the top-selling herbal supplement in mainstream retail outlets in the United States. Sales of horehound supplements, which include cough drops and lozenges with horehound as the primary ingredient, totaled $125,468,033 in 2016! Horehound has lead the list for the last four years.
Horehound is a bitter herb that has had medicinal uses dating back to the Egyptians. It has been used in the treatment of cough and other lung ailments, as a digestive aid and appetite stimulant, and as a remedy for motion sickness.
Below I examine the available medical and scientific literature to see if any of these claims can withstand careful scrutiny.
What Is Horehound?
Horehound is a perennial plant in the mint family, Lamiaceae. There are at least three different forms of horehound- white horehound (Marrubium vulgare), black horehound (Ballota nigra), and water horehound (Lycopus americanus, also known as bugleweed). For this article, I will be referring primarily to white horehound.
Horehound is native to Europe, northern Africa, and southwestern and central Asia. It has also been naturalized to many other places, including most of North and South America.
Since ancient Egypt, white horehound has been used to help remove mucus from the lungs or throat. Horehound is first mentioned in the written literature by Aulus Cornelius Celsus, an encyclopedist in his medical work, De Medicina. 
“But if the cough is dry and very troublesome, it is relieved by taking a cup of dry wine, … further, there is need to swallow a little of the best laser [a Roman gum-resin], to take juice of leeks or horehound…”
Horehound is currently used in Europe and can be found in many European herbal cough remedies (such as Ricola®). In 1989, the U.S. Food and Drug Administration concluded that horehound was not effective as an expectorant in nonprescription (“over-the-counter” or OTC) cough and cold medicines and should not be included as an active ingredient in such products.
The parts of white horehound used medicinally are the dried leaves and flowering tops. Processed white horehound contains 0.3-1% of the bitter compound marrubiin, diterpene alcohols, alkaloids, bitter lactone, flavonoids, saponin, sterols, tannins, and vitamin C, and 0.06% of a volatile oil.
Marubiin is considered the main bioactive ingredients in horehound. It is a diterpenoid lactone. Terpenoids are organic compounds produced by a variety of plants (and a few animals) which are used for their aromatic qualities. The scent of eucalyptus, cinnamon, clove, and ginger are attributed to the terpenoids in them.
As horehound has been used in the treatment of cough, as a digestive aid and appetite stimulant, as a treatment for diabetes, and as an analgesic, scientists have tested marrubiin for pharmacologic properties that could justify its use for these conditions.
Horehound is available fresh, dried, powdered, in capsules, as an extract, or as a pressed juice. It can be used in to make tea or added to sugar to make candy.
Is There Any Research?
As of January 2018, the biomedical literature (PubMed.gov) contains only 205 articles about horehound dating back to 1947. Most of these research studies involve experiments with laboratory animals (mostly rats or mice) or “test tubes” (in vitro studies). When I searched for “Marrubium vulgare,” I found 122 papers, many of which were already cited under “horehound.” There were 30 papers on marrubiin. Of all the articles mentioning horehound, only one study appears to involve human subjects.
Turning to the U.S. government’s database of clinical research trials (ClinicalTrials.gov), there are no clinical trials available for horehound, Marrubium vulgare, or marrubiin.
By comparison, I searched for guaifenesin, the active ingredient found in many nonprescription or “over-the-counter” (OTC) cold medicines and found almost 900 articles in PubMed. There were 120 studies cited about clinical use of guaifenesin, and 38 clinical studies on ClinicalTrials.gov.
The extremely limited number of scientific studies (with only one in humans) makes it difficult to do much more than speculate about the potential health benefits of horehound. Much more research is needed.
Does Horehound Relieve Coughs?
Horehound has been used as a folk remedy for cough for thousands of years. It is believed to be an expectorant (a substance that promotes the secretion of mucus in the air passages) and a demulcent (a substance that relieves irritation of the mucous membranes).
I could find only one study of horehound (in this case black horehound) involving cough suppression in mice.
As previously mentioned, the FDA banned horehound from OTC, nonprescription cough and cold medications produced in the U.S. because there was insufficient evidence to support its efficacy. In Europe, however, there are different rules concerning the regulation and use of herbal medicinal products. Specifically, there is a regulatory pathway called “Traditional use registration.” For an herbal medicine to be approved under this pathway, it must meet the following criteria:
- No clinical tests and trials on safety and efficacy are required so long as sufficient safety data and plausible efficacy are demonstrated
- Involves assessment of mostly bibliographic (literature-based) safety and efficacy data
- Must have been used for at least 30 years, including at least 15 years within the EU
- Intended use is without the supervision of a medical practitioner and not administered by injection
The European Medicines Agency (EMA) , as well as the German Commission E recognize the traditional use of horehound as an “expectorant (a medicine to help bring up phlegm) in patients with cough associated with a cold.”
On a personal note, as someone who has popped more than a few Ricola cough drops in my time, I was interested to find that horehound is just one of ten herbs used in their cough drops. The others are lemon balm, elder, peppermint, mallow, sage, thyme, lime flowers hyssop, and wild thyme. The amount of each in their preparations is not available.
There are no clinical studies that prove the efficacy of horehound as a cough suppressant or expectorant. It has been approved by the EMA and German Commission E for use in colds under “Traditional Use” regulatory rules.
Does It Aid Digestion?
Horehound is considered a bitter herb. Bitters stimulate the bitter receptors in the taste buds at the back of the tongue. (Note: Researchers are finding that bitter taste receptors may also be found in the stomach, gut, and even in the human respiratory tract.) Traditionally, bitters are considered digestive aids- stimulating saliva and increasing gastric juices. The mechanisms by which bitters can do this are still under investigation. The two main models are:
- Cephalic Vagal Reflex: stimulation of bitter receptors in the mouth act reflexively to increase saliva and stimulation of the vagus nerve to the digestive organs. Evidence used to support this model include changes in heart rate and blood flow (also controlled by the vagus nerve). 
- Local Reflex: stimulation of bitter receptors in the both the mouth and in the GI tract act locally to increase digestive secretions. Evidence supporting this model is that there are some bitters that do not affect heart rate. 
The herbs that are typically used in these studies are gentian and wormwood. I could find no studies done with horehound.
Horehound may have some antispasmodic effects. A study by Schlemper et al stimulated the smooth muscle from the intestines removed from guinea pigs and rats with the of neurotransmitters acetylcholine, histamine and bradykinin. The amount of response was measured. The intestinal material was tested with these chemicals again, this time with the addition of varying doses of an extract from horehound. The responses where significantly diminished, and were dose related.
Lastly, researchers have investigated horehound as a gastroprotective (anti-ulcer) agent. A study by Paula de Oliveira et al. looked at induced stomach ulcers in mice. Mice were given either a control substance, an anti-ulcer drug (omeprazole or cimetidine), or various doses of an extract from marrubium vulgare. The mice were then subjected to either alcohol or indomethacin (a non-steroidal anti-inflammatory drug) to induce ulcers. The mice were then sacrificed, and their stomachs examined for ulcerations and for gastric secretions and mucus present in the stomach. In both scenarios, the mice treated with marrubium extract showed a decrease in ulcerations, a decrease in acidity, and an increase in mucus in the stomach. At some doses of the extract, the results were comparable to those obtained with omeprazole or cimetidine.
Although there are a tiny number of studies that show that horehound might aid digestion or be gastroprotective, there is no evidence or nor studies done in humans to support these claims. Further research should to done.
Does It Cure Motion Sickness?
Although black horehound (Ballotta nigra) has been a traditional remedy for motion sickness, there are no scientific studies to support this claim.
There is no scientific evidence to support the claim that horehound can cure motion sickness.
Does It Help Bronchitis?
Bronchitis is a condition in which the bronchial tubes, which carry air to your lungs, become inflamed. People who have bronchitis often have a cough that brings up mucus. Bronchitis also may cause wheezing (a whistling or squeaky sound when you breathe), chest pain or discomfort, a low fever, and shortness of breath.
As mentioned above, The European Medicines Agency (EMA) , as well as the German Commission E recognize the traditional use of horehound as an expectorant, although this is based on its traditional usage. An expectorant might be useful in patients with bronchitis.
According to Villanueva and Esteban , horehound tea is used as an anti-asthmatic in Sardinia (Italy). A study of 5 asthmatic patients by Bellero, Sotgiu, and Piu  believed its therapeutic effect may be due to flavonoids in alcoholic extracts of the drug. The full text of this study is not available.
A search of PubMed on horehound (or Marrubium vulgare) and bronchitis did not reveal any studies on horehound and bronchitis in vitro, in animal trials or in humans.
There are no clinical studies to support the use of horehound in the treatment of bronchitis.
Is It an Appetite Stimulant?
As a bitter herb, horehound might be assumed to have appetite stimulant effects- stimulating saliva and increasing gastric juices- like other bitter herbs, such as gentian and wormwood. However, there are no clinical trials or animal studies to support this.
Just as with its expectorant effects, horehound is approved for use as a temporary appetite stimulant by the EMA and German Commission E under the traditional use guidelines.
Although approved for use as a temporary aid for appetite, there are no clinical trials to support the use of horehound as an appetite stimulant.
Is Horehound (and or Supplementation) Safe?
No serious adverse events with horehound were reported by Izzo in a 2016 review of herbal remedies, although there is a limited number of safety studies, and many were performed on animal subjects.
No clinical trials on children, older adults, and pregnant or breastfeeding women have been done, and the use of horehound in these groups is not recommended.
As horehound may increase gastric secretions, patients with peptic ulcer disease should contact their physician before using preparations with M. vulgares.
Excessive use of horehound has been associated with an increased risk of heart arrhythmia.
Possible interactions :
- Because horehound may work to lower blood pressure, caution should be used in those taking blood pressure medication.
- Horehound’s possible effect as a diuretic should lead those on diuretic medication or water pills to take caution.
- Horehound may lower blood sugar. Those using medications that affects blood sugar should consult their health practitioner.
- Because horehound may work as an expectorant, it may change or increase the effect of cold medications.
- Horehound may increase the effect of laxative products or cholesterol-lowering medications.
- Because horehound contains glycosides and estrogen-like chemicals, those taking heart medications or hormone therapy should use caution as well.
Based on my research, I see no reason to abandon using Ricola® when I need some relief from coughing and dry throat. But horehound is only one of 10 herbal ingredients in these drops.
Although not approved as effective as an expectorant or cough suppressant by the U.S. FDA, horehound is approved for this use, and as an appetite stimulant, in Europe as it meets the criteria for “Traditional use registration” for nonprescription, herbal medicines.
All other health claims for horehound should be treated with great skepticism because there is little or no scientific evidence to support these claims. And based on this largely negative, weak or non-existent evidence in the biomedical literature, there appears to be little motivation to conduct larger and more definitive studies.