Supriya Ghosh (Editor)

Medicinal plants

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Medicinal plants

Medicinal plants, medicinal herbs, or simply herbs have been identified and used from prehistoric times. Plants make many chemical compounds for biological functions, including defence against insects, fungi and herbivorous mammals. Over 12,000 active compounds are known to science. These chemicals work on the human body in exactly the same way as pharmaceutical drugs, so herbal medicines can be beneficial and have harmful side effects just like conventional drugs. However, since a single plant may contain many substances, the effects of taking a plant medicine can be complex.

Contents

The earliest historical records of herbs are found from the Sumerian civilisation, where hundreds of medicinal plants including opium are listed on clay tablets. The Ebers Papyrus from ancient Egypt describes over 850 plant medicines, while Dioscorides documented over 1000 recipes for medicines using over 600 medicinal plants in De materia medica, forming the basis of pharmacopoeias for some 1500 years. Drug research makes use of ethnobotany to search for pharmacologically active substances in nature, and has in this way discovered hundreds of useful compounds. These include the common drugs aspirin, digoxin, quinine, and opium. The compounds found in plants are of many kinds, but most are in four major biochemical classes, the alkaloids, glycosides, polyphenols, and terpenes.

Medicinal plants are widely used to treat disease in non-industrialized societies, not least because they are far cheaper than modern medicines. The annual global export value of pharmaceutical plants in 2012 was over US$2.2 billion.

Prehistoric times

Plants, including many now used as culinary herbs and spices, have been used as medicines from prehistoric times. Spices have been used partly to counter food spoilage bacteria, especially in hot climates, and especially in meat dishes which spoil more readily. Angiosperms (flowering plants) were the original source of most plant medicines. Human settlements are often surrounded by weeds useful as medicines, such as nettle, dandelion and chickweed. Humans were not alone in using herbs as medicines: some animals such as non-human primates, monarch butterflies and sheep ingest medicinal plants to treat illness. Plant samples from prehistoric burial sites are among the lines of evidence that Paleolithic peoples had knowledge of herbal medicine. For instance, a 60 000-year-old Neanderthal burial site, "Shanidar IV", in northern Iraq has yielded large amounts of pollen from 8 plant species, 7 of which are used now as herbal remedies, A mushroom was found in the personal effects of Ötzi the Iceman, whose body was frozen in the Ötztal Alps for more than 5,000 years. The mushroom was probably used to treat whipworm.

Ancient times

In ancient Sumeria, hundreds of medicinal plants including myrrh and opium are listed on clay tablets. The ancient Egyptian Ebers Papyrus lists over 800 plant medicines such as aloe, cannabis, castor bean, garlic, juniper, and mandrake. From ancient times to the present, Ayurvedic medicine as documented in the Atharva Veda, the Rig Veda and the Sushruta Samhita has used hundreds of pharmacologically active herbs and spices such as turmeric, which contains curcumin. The Chinese pharmacopoeia, the Shennong Ben Cao Jing records plant medicines such as chaulmoogra for leprosy, ephedra, and hemp. This was expanded in the Tang Dynasty Yaoxing Lun. In the fourth century BC, Aristotle's pupil Theophrastus wrote the first systematic botany text, Historia plantarum. In the first century AD, the Greek physician Pedanius Dioscorides documented over 1000 recipes for medicines using over 600 medicinal plants in De materia medica; it remained the authoritative reference on herbalism for over 1500 years, into the seventeenth century.

Middle Ages

In the Early Middle Ages, Benedictine monasteries preserved medical knowledge in Europe, translating and copying classical texts and maintaining herb gardens. Hildegard of Bingen wrote Causae et Curae ("Causes and Cures") on medicine. In the Islamic Golden Age, scholars translated many classical Greek texts including Dioscorides into Arabic, adding their own commentaries. Herbalism flourished in the Islamic world, particularly in Baghdad and in Al-Andalus. Among many works on medicinal plants, Abulcasis (936–1013) of Cordoba wrote The Book of Simples, and Ibn al-Baitar (1197–1248) recorded hundreds of medicinal herbs such as Aconitum, nux vomica, and tamarind in his Corpus of Simples. Avicenna included many plants in his 1025 The Canon of Medicine. Abu-Rayhan Biruni, Ibn Zuhr, Peter of Spain, and John of St Amand wrote further pharmacopoeias.

Early Modern

The early modern period saw the flourishing of illustrated herbals across Europe, starting with the 1526 Grete Herball. John Gerard wrote his famous The Herball or General History of Plants in 1597, based on Rembert Dodoens, and Nicholas Culpeper published his The English Physician Enlarged. Many new plant medicines arrived in Europe as products of Early Modern exploration and the resulting Columbian Exchange, in which livestock, crops and technologies were transferred between the Old World and the Americas in the 15th and 16th centuries. Medicinal herbs arriving in the Americas included garlic, ginger, and turmeric; coffee, tobacco and coca travelled in the other direction. In Mexico, the sixteenth century Badianus Manuscript described medicinal plants available in Central America.

Phytochemistry

Many plants produce chemical compounds for defence against herbivores. These are often useful as drugs. The major classes of pharmacologically active phytochemicals are described below.

Alkaloids

Alkaloids are bitter-tasting chemicals, very widespread in nature, and often toxic. There are several classes with different modes of action as drugs, both recreational and pharmaceutical. Medicines of different classes include atropine, scopolamine, and hyoscyamine (all from nightshade), berberine (from plants such as Berberis and Mahonia), caffeine (Coffea), cocaine (Coca), ephedrine (Ephedra), morphine (opium poppy), nicotine (tobacco), psilocin (many psilocybin mushrooms), reserpine (Rauwolfia serpentina), quinidine and quinine (Cinchona), vincamine (Vinca minor), and vincristine (Catharanthus roseus).

Glycosides

Anthraquinone glycosides are found in the laxatives senna, rhubarb and Aloe.

The cardiac glycosides are powerful drugs from plants including foxglove and lily of the valley. They include digoxin and digitoxin which support the beating of the heart, and act as diuretics.

Polyphenols

Polyphenols of several classes are widespread in plants. They include the colourful anthocyanins, hormone-mimicking phytoestrogens, and astringent tannins. In Ayurveda, the astringent rind of the pomegranate is used as a medicine, while polyphenol extracts from plant materials such as grape seeds are sold for their potential health benefits despite the lack of evidence.

Plants containing phytoestrogens have been used for centuries to treat fertility, menstrual, and menopausal problems. Among these plants are Pueraria mirifica, kudzu, angelica, fennel, and anise.

Terpenes

Terpenes and terpenoids of many kinds are found in resinous plants such as the conifers. They are strongly aromatic and serve to repel herbivores. Their scent makes them useful in essential oils, whether for perfumes such as rose and lavender, or for aromatherapy.

Cultivation

Medicinal plants demand intensive management. Different species each require their own distinct conditions of cultivation. The World Health Organization recommends the use of rotation to minimise problems with pests and plant diseases. Cultivation may be traditional or may make use of conservation agriculture practices to maintain organic matter in the soil and to conserve water, for example with no-till farming systems. In many medicinal and aromatic plants, plant characteristics vary widely with soil type and cropping strategy, so care is required to obtain satisfactory yields.

Usage

Plant medicines are ubiquitous in pre-industrial societies, while some 7,000 conventional medicines such as aspirin, digitalis, opium, and quinine derive directly from traditional plant medicines, accounting for around a quarter of the modern pharmacopoeia. They are in general far cheaper, and many can be home-grown or picked for free. Further, pharmaceutical companies have made use of the herbal knowledge of indigenous peoples around the world to search for new drug candidates. In India, where Ayurveda has been practised for centuries, herbal remedies are the responsibility of a government department, AYUSH, under the Ministry of Health & Family Welfare. Traditional Chinese medicine makes use of a wide variety of plants, among other materials and techniques. Complementary and alternative medicines including herbal therapy are widely used in the Western world, for example by over a third of Americans.

Plant medicines including opiates, cocaine and cannabis have both medical and recreational uses. Different countries have at various times made some uses of drugs illegal, partly on the basis of the risks involved in taking psychoactive drugs.

Effectiveness

Plant medicines have often not been tested systematically, but have come into use informally over the centuries. By 2007, clinical trials had demonstrated potentially useful activity in nearly 16% of herbal medicines; there was limited in vitro or in vivo evidence for roughly half the medicines; there was only phytochemical evidence for around 20%; 0.5% were allergenic or toxic; and some 12% had effectively never been studied scientifically. According to Cancer Research UK, "there is currently no strong evidence from studies in people that herbal remedies can treat, prevent or cure cancer". Modern knowledge of medicinal plants is being systematised in the Medicinal Plant Transcriptomics Database, which provides a sequence reference for the transcriptome of some thirty species. Hundreds of compounds have been identified using ethnobotany, investigating plants used by indigenous peoples. There is renewed interest in the discovery of therapeutically useful substances from medicinal plants.

Safety

Plant medicines can cause adverse effects and even death, whether by side-effects of their active substances, by adulteration or contamination, by overdose, or by inappropriate prescription. Many such effects are known, while others remain to be explored scientifically. There is no reason to presume that because a product comes from nature it must be safe: the existence of powerful natural poisons like atropine and nicotine shows this to be untrue. Further, the high standards applied to conventional medicines do not always apply to plant medicines, and dose can vary widely depending on the growth conditions of plants: older plants may be much more toxic than young ones, for instance. Pharmacologically active plant extracts can interact with conventional drugs, both because they may provide an increased dose of similar compounds, and because some phytochemicals interfere with the body's systems that metabolise drugs in the liver including the cytochrome P450 system, making the drugs last longer in the body and have a more powerful cumulative effect. Plant medicines can be dangerous during pregnancy. Since plants may contain many different substances, plant extracts may have complex effects on the human body.

Quality

Herbal medicines and supplements are not always tested for purity, and there is some concern about adulteration and inclusion of allergens such as soy and wheat in some products.

References

Medicinal plants Wikipedia


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