Demulcent antitussives

Antitussives act either centrally on the medullary cough center of the brain or peripherally at the site of irritation. Although some of the best centrally active antitussives (e.g., codeine) are plant products, they are subject to abuse and are not available for self-selection. Consequently, they are not discussed here.

Certain volatile oils obtained from herbs are incorporated into a sugar base and marketed in the form of lozenges to suppress coughs. Some of the more popular oils used for this purpose include anise, eucalyptus, fennel, peppermint, and thyme. Cough drops flavored with these oils apparently function by stimulating the formation and secretion of saliva, which produces more frequent swallowing and thereby tends to suppress

the cough reflex. 21 However, the real therapeutic utility of volatile-oil-containing herbs in treating conditions associated with colds and flu is their expectorant action. The effective herbal expectorants will be considered following the antitussives.

Two natural herbal products, camphor and menthol, are used topi- cally as antitussives. Ointments containing these drugs are rubbed on the throat and chest, and they may also be used in steam inhalers. The aromatic vapor that is inhaled from these applications has a local anes- thetic action on the lungs and throat, which suppresses the cough reflex. Menthol is used in cough drops for this same purpose.

The antitussive effect of many herbs results from the content of mucilage, which exerts a demulcent or protective action. Mucilages are hydrophilic colloids that, in the presence of water, tend to form viscous solutions—or tacky gels. When consumed, usually in the form of a tea, they form a protective layer over the mucous membrane of the pharynx, larynx, and trachea, thereby preventing mechanical irritation of the recep- tors there and preventing the cough reflex.

Because the mucilage is not absorbed and its action is essentially a mechanical one, it does not produce untoward side effects. However, some mucilage-containing herbs possess additional constituents that are toxic. This is the case with one of the long-used herbal antitussives, coltsfoot (the

Chapter five: Respiratory tract problems

83 leaves of Tussilago farfara L. of the family Asteraceae). Although coltsfoot

has useful cough-protective properties, its use cannot be recommended because it also contains toxic pyrrolizidine alkaloids (PAs). 22

The following mucilage-containing antitussive herbs may be employed more or less interchangeably and are listed in alphabetical order.

Iceland moss Not a higher plant, but rather a lichen—that is, an alga and a fungus growing in symbiotic association—Iceland moss is obtained from Cetraria islandica (L.) Ach. of the family Parmeliaceae. Commercial supplies of this foliaceous lichen are obtained primarily from Scandinavia and central Europe. It contains about 50 percent of a mixture of mucilaginous polysac- charides, principally lichenin and isolichenin. Iceland moss is consumed in the form of a decoction prepared from 1–2 heaping teaspoonfuls of herb

and 150 mL of water. Drink one cup three times a day. 23 Total daily dose is 4–6 g of plant material. The German Commission E has found Iceland moss effective for the treatment of irritations of the mouth and throat and associated dry cough. 24

Finnish scientists warn against utilizing Iceland moss in large quantities over an extended period of time. It has long been used as an emer- gency food in that country; however, in recent years, the lead content of the lichen has increased to the point (30 mg per kilogram of dry weight) at

which this practice can no longer be considered safe. 25 Although the rela- tively small amounts used occasionally for the treatment of cough prob- ably pose negligible risk, it is nevertheless a concern of which consumers should be aware.

Marshmallow root This herb consists of the dried root, deprived of the brown outer corky layer, of Althaea officinalis L. (family Malvaceae). It contains 5–10 percent of mucilage and is consumed in the form of a tea, 1–2 teaspoonfuls in 150 mL

of water (daily dose: 6 g), for its antitussive effect. 21 In Europe, the leaves of the plant, as well as the leaves and flowers of the common mallow, Malva sylvestris L., and related species and subspecies, are all employed simi- larly. Commission E has declared them all to be effective demulcents. 24

Mullein flowers The flowers of several species of mullein—Verbascum thapsus L., V. densi- florum Bertol., and V. phlomoides L. (family Scrophulariaceae)—all contain about 3 percent of a mucilage useful in the treatment of throat irritations and cough. The flowers (3–4 teaspoonfuls) are used to prepare 150 mL of

tea, which may be drunk several times daily. 21 Approved by Commission

E for the treatment of respiratory catarrh, the herb also has some expectorant activity. 26

84 Tyler's herbs of choice: The therapeutic use of phytomedicinals Plantain leaf

Fresh or dried leaves of the English plantain, Plantago lanceolata L. (family Plantaginaceae), have a worldwide reputation as a soothing cough sup- pressant. This action is attributed primarily to the approximately 6 percent of mucilage found in the plant material; tannins and bitter principles may contribute as well. The herb is also employed for inflammatory conditions of the oral cavity as well as to treat various skin inflammations.

Plantain’s effectiveness in these latter conditions is due in part to its mucilage content; in addition, two iridoid glycosides, aucubin and catapol, almost certainly play a role, at least under certain conditions. When the sap is expressed from the fresh leaves, the glycosides are hydrolyzed, and the residual aglycones exert a strong antibacterial effect. This accounts for the folkloric use of the fresh crushed leaves as an anti-inflammatory and wound-healing agent. Of course, the antibacterial products are not pres- ent in the infusions customarily used to relieve coughs because boiling water inactivates the hydrolytic enzyme. 27

German Commission E has found plantain safe and effective as a soothing demulcent, astringent, and antibacterial. 28 It is customarily administered as a tea prepared from 3–4 teaspoonfuls of the herb and 150 mL of boiling water.

Slippery elm Indians and early settlers of North America valued the inner bark of the slippery elm, Ulmus rubra Muhl. (family Ulmaceae), as a poultice and soothing drink. The bark of this large tree, native to the eastern and central United States, contains large quantities of a viscid mucilage that acts

as an effective demulcent and antitussive. 29 Although the herb may be consumed in the form of a tea, a number of throat lozenges containing it are commercially available. These are the preferred dosage form for the treatment of cough and minor throat irritations because they provide a sustained release of the mucilage to the pharynx.

This native American herb has not seen widespread usage elsewhere, so European authorities have not commented on its safety and efficacy. Some measure of its utility may be gathered from the fact that it was listed

in the official compendia (USP and NF) from 1820 to 1960. 30 The FDA has declared it to be a safe and effective oral demulcent.

Expectorants

Prolonged irritation of the bronchioles results in an increase in the muco- protein and acidic mucopolysaccharide content of their secretions and

a concomitant increase in the viscosity of the mucus and other fluids. This and several related factors reduce the ability of ciliary movement and coughing to move the thickened secretions toward the pharynx.

Chapter five: Respiratory tract problems

85 Symptomatic therapy with expectorants has the objective of reducing the

viscosity of these secretions so that the loosened material may be elimi- nated from the system, eventually, by expectoration.

The action of the so-called nauseant–expectorant herbs that contain alkaloids results primarily from their action on the gastric mucosa. This provokes a reflex stimulation of the vomiting center in the brain via the vagus nerve, which leads to an increase in secretion of the bronchial glands. Volatile-oil type expectorant herbs, on the other hand, exert a direct stimulatory effect on the bronchial glands by means of local irritation. Saponin-containing expectorant herbs function by reducing the surface tension of the secretions, facilitating their separation from the

mucous membranes. 31 Of course, some expectorant herbs combine two or more of these effects. For example, the saponin-containing senega root also possesses nauseant–expectorant properties.

Use of expectorants is based primarily on tradition. Subjectively, they appear to be effective for the treatment of irritative, nonproductive coughs associated with a small amount of secretion. Substantial proof of their therapeutic utility is lacking. Nevertheless, they form a significant group of herbal remedies, and some appear to be of value. In the following dis- cussion, they are classified on the basis of their mode of action: (1) nauseant–expectorants, (2) local irritants, and (3) surface-tension modifiers. The classification is imprecise because the function of many of the herbs is incompletely understood and some play multiple roles.

Demulcent antitussives