Herbal Medicines

Herbal medicines are medicines made from plants. A survey of some 259 of the most widely used plants in western herbal medicine in Australia found that the vast majority are flowering plants (angiosperms). Approximately one-third of the species belong to just five botanical families: the daisy family (Asteraceae), mint family (Lamiaceae), rose family (Rosaceae), carrot family (Apiaceae) and legume family (Fabaceae). The study also surveyed the biogeographical origin of medicinal species and the morphological plant parts used for medicinal purposes. These results are shown in Table Biogeographical origin of 259 species used in western herbal medicine and Table Morphological plant part used for medicine respectively.

Table Biogeographical origin of 259 species used in western herbal medicine (after Wohlmuth 2002)

Europe/Europe and parts of Asia37.4%
Asia19.3%
Africa3.0%
North America21.6%
South America3.5%
Pacific (incl. Australia)1.2%
Native to several continents14.0%

Table Morphological plant part used for medicine (after Wohlmuth 2002)

Plant part used
Aerial parts37.8%
Underground parts27.8%
Fruit/seed13.9%
Bark8.5%
Flower4.6%

Herbal medicines are relatively crude extracts of medicinal plants and are characterised by containing a range of chemical constituents, several of which may contribute to the pharmacological effects of the medicine. This contrasts with plant-derived and synthetic pharmaceutical drugs, which typically contain a single, purified active compound, as shown in Table Chemical constituents of herbal medicines and pharmaceutical drugs.

Table Chemical constituents of herbal medicines and pharmaceutical drugs

Therapeutic agentsChemical constituent(s)
Herbal medicines
Meadowsweet {Filipendula ulmaria), flowers and leafFlavonoids (incl. different glycosides of quercetin and kaempferol); phenolic glycosides (incl. salicylaldehyde primveroside and methyl salicylate primveroside); essential oil (incl. salicylaldehyde, phenylethyl alcohol, methyl salicylate); tannins (incl. rugosin-D); and others
Licorice (Clycyrrhizaglabra), rootTriterpenoid saponins (incl. glycyrrhizin); glycyrrhetinic acid; flavonoids (incl. flavanones, chalcones and isoflavonoids); sterols ()
Plant-derived pharmaceutical drugs
Lanoxin®Digoxin
Oncovin®Vincristine sulphate
Synthetic pharmaceutical drugs
Telfast®Fexofenadine
Zocor®Simvastatin

Plants as sources of pharmacologically active compounds

It has been known for centuries that many plants contain compounds with powerful pharmacological effects. Most pharmacologically active plant constituents are so-called secondary metabolites, such as alkaloids, saponins and coumarins. These secondary metabolites are compounds that do not appear to be essential to the short-term survival of a plant, in contrast to primary metabolites such as carbohydrates, lipids, proteins and nucleic acids. The first secondary metabolites to be isolated from plants in the early nineteenth century were all alkaloids with powerful pharmacological effects: morphine (1816), strychnine (1817), atropine (1819) and quinine (1820) (Mann 1992). In the twentieth century many pharmaceutical drugs were based on plant compounds. In 1985 it was estimated that 25 per cent of pharmaceutical drugs dispensed in the USA between 1959 and 1980 were based on active principles from plants.

Compounds extracted from plants are still being used in pharmaceutical drugs. Well-known examples are the analgesic alkaloid morphine, extracted from the opium poppy (Papaver somniferum), and the cardiac glycoside digoxin, extracted from the Grecian foxglove (Digitalis lanata). Many other pharmaceutical drugs are derived from plant compounds, either by chemical modification of a plant compound or by having their synthetic structure modelled on plant compounds. The steroidal drugs are an example of the former, being produced semi-synthetically from steroidal plant precursors, mostly from soybeans (Glycine max) or yams (Dioscorea spp.). The world’s most successful pharmaceutical drug, acetylsalicylic acid (Aspirin®), was developed from naturally occurring plant salicylates. It was first marketed in 1899 by the German pharmaceutical company Bayer.

Pharmacologically active plant compounds are typically small molecules which, like other drugs, can interact with protein targets called receptors in the human body. The main way in which a herbal medicine differs from a plant-derived pharmaceutical drug is that it contains multiple chemical compounds, many of which may contribute to the effect of the remedy. In contrast, pharmaceutical drugs (plant derived or not) normally consist of a single, purified chemical compound which will interact with one specific receptor. For example, a tincture of peppermint is a herbal medicine, whereas morphine, a pure plant compound, is a pharmaceutical drug. Table Differences between herbal medicines and pharmaceutical drugs lists some characteristic differences between herbal medicines and pharmaceutical drugs.

Table Differences between herbal medicines and pharmaceutical drugs

Herbal medicinesPharmaceutical drugs
Relatively crude plant extractsSynthetic or highly purified
Multiple active compoundsSingle active compound
Potentially multiple targets of actionSingle target of action

Types of herbal medicines

Herbal medicines are used topically and orally and come in a variety of forms. The simplest form of herbal medicine is a herbal tea made from the dried herb. Teas may be prepared either as infusions, where the herb is steeped in freshly boiled water, or as decoctions, where the herb is simmered in boiling water. Which is the more appropriate method depends on the constituents and the nature of the plant material. The therapeutic effects of medicinal teas should not be underestimated provided good-quality herb is used in an appropriate strength and dose.

Apart from dried herbs, the majority of retail preparations are in solid dosage forms such as tablets or capsules, whereas preparations for practitioner dispensing are more commonly liquid extracts. These extracts are made by extraction of the plant material using a mixture of ethanol and water as solvent. More dilute forms of liquid extracts are often called tinctures. Another type of liquid extract uses a glycerol-water mixture as the extraction medium; the resultant extracts are called glycerol extracts or ‘glycetracts’ and have the advantages of being alcohol free and sweet tasting. Glycerol is not, however, a good universal solvent for most plant constituents and in most cases a glycerol extract will be inferior to an equivalent ethanol-water extract. Fresh juices of a variety of medicinal plants are also available in botded form in some countries.

Tablets and capsules may contain powdered dried herb, but increasingly solid or semi-solid extracts are used for these dosage forms. Such extracts are often prepared by drying liquid extracts. In recent years an increasing number of highly concentrated extracts have become available. Some of these have high raw material (herb) to extract ratios. The widely used ginkgo leaf extract, for example, has an average herb to extract ratio of 50 to 1, that is, 50 kg of ginkgo leaf is used to produce 1 kg of extract. It is obvious that in the case of highly concentrated preparations, the extraction must be highly selective. In the example of the ginkgo leaf extract only 2 per cent of the raw material ends up in the extract; the rest is discarded. Concentrated extracts are often prepared by way of sophisticated extraction methods such as supercritical fluid extraction or the use of repeated extraction with multiple solvents. What defines these extracts as still being herbal and not pharmaceutical drugs is the fact that they retain a high degree of chemical complexity.

Most of these concentrated extracts are also standardised, which means that standardised extraction and manufacturing processes have been employed in their production. In the case of standardised extracts of high quality, the process of standardisation includes agronomic practices such as seed selection and cultivation, as well as the application of quality assurance systems that effectively monitor all steps of production, from the growing plant to the finished extract. Standardised extracts are also characterised by containing quantified levels of one or more marker compounds. The level of a marker compound is usually given as a minimum content or as a range. Undesirable constituents may also be specified by way of a maximum level (e.g. ginkgolic acids in ginkgo leaf extract). Marker compounds may be compounds with known pharmacological activity or may be compounds that are characteristic of the particular plant species.

Essential (or volatile) oils are chemically complex constituents of many medicinal plants. They can be selectively extracted by steam distillation. The use of pure essential oils in herbal medicine is very limited, but examples of oils that are used therapeutically are peppermint oil and tea tree oil (the steam-distilled oils of Mentha x piperita and Melaleuca alternifolia, respectively).

Quality issues in herbal medicine

There are numerous quality issues pertinent to herbal medicines. These range from basic issues of correct botanical identification, harvest, drying and storage methods, to issues of good manufacturing practice and stability (see Table Parameters that impact on the quality of herbal medicines and measures taken to control them). The most obvious limiting factor to the quality of a product is the quality of the raw material from which it is made. Most herbal medicines are made from dried plant material and the timing and methods of harvest, as well as the drying and storage conditions, are critical.

It is fair to say that there is enormous variation in the quality of herbal medicines currently on the market in most countries. It is also evident that regulating authorities as well as responsible sectors of the industry are making concerted efforts towards improving quality standards of herbal medicines.

Table Parameters that impact on the quality of herbal medicines and measures taken to control them

ParameterMeasure
Botanical identity of raw materialCorrect botanical identification
Correct medicinal part(s) of plantCorrect morphological identification
Quality of raw materialGood agricultural practice; correct harvest and post-harvest handling; tests for marker compounds, contaminants (heavy metals, micro-organisms, pesticide residues)
Extraction and manufacturingGood manufacturing practice; use of appropriate methodology; test for marker compounds
StabilityStability testing on extract/product
Quality useAppropriate packaging and labelling (indications, dosage, warnings)