Phytotherapy, also called herbal medicine or herbalism, is the use of plants or herbs to treat diseases, to alleviate pain, or maintain good health. In the U.K., it is defined as an alternative method of "Group 1", or Professionally Organised Alternative Therapies. 
It is a member of the class of biologically based health practices. Specific types of phytotherapy are often traditional medicine of different cultures, and use of an indigenous knowledge system (IKS). Herbal plant preparations are part of traditional Chinese medicine. There are many European traditional uses.
Phytochemicals versus plant extracts
Traditional herbalists tend to emphasize the use of either the entire plant, or a simple extract from it, such as a water or ethanol extract, which explicitly do not attempt to extract specific chemicals. Part of the holistic tradition in many herbal approaches is that Nature provides a complete remedy, and the many compounds in a medicinal plant contribute to its beneficial effects.
Holistic versus evolutionary models
Evolutionary botanists, however, have questioned why a plant would evolve to have a wide range of substances that specifically benefit humans in an abnormal physiological state. It is not at all unreasonable that a plant might have an evolutionary advantage if it starts to generate a chemical that inhibits some parasite, and this chemical might well inhibit a related parasite in animals.
It is less clear why the plant might develop a spectrum of phytochemicals with a synergistic effect, but it is certainly not impossible; there are a number of cases, in medicine, where a prescribed drug is a deliberate combination. For example, several drugs contain an active beta-lactam antibiotic (e.g., ampicillin) with another chemical (e.g., clavulanic acid) that protects the antibiotic from a bacterial resistance mechanisms that destroys the antibiotic.
The problem of consistency
Few phytotherapies have a longer history, with real benefit, than the use of the foxglove plant, Digitalis purpurea for cardiac disease. The plant, which is extremely attractive and is often cultivated as an ornamental, contains potent digitalis glycosides that support the activity of the heart. These phytochemicals, however, have a narrow margin between the effective dose and the toxic dose. Foxglove plants, grown in different places and times, may have different concentrations of the glycosides, so water extraction (i.e., a tea or tisane), done in exactly the same manner, may produce more or less active phytochemical because the individual plants vary.
For a number of reasons, it has become standard practice to synthesize many plant-derived drugs. One of the most common reasons is to be able to measure the concentration of a single active chemical, and make certain tablets containing it have a constant concentration.
Different methods of preparation can yield different concentrations. With the foxglove, for example, extracting with alcohol will yield a much higher concentration than will water extraction. Of course, even with water extraction, the concentration can vary if the raw plant is in contact with the water for longer or shorter times. Temperature also affects the concentration extracted into water.
There is no single paradigm for phytotherapy outside the narrow definition of extracting phytochemicals for conventional medicine. At the very least, there are different traditional Chinese, ayurvedic, and Western paradigms to primary herbalism.
There are three major sources of information in traditional Western herbalism. Depending on the practitioner, they may be used together, and, indeed, in combination with biomedical approaches. They are empiricism, theories, and revelation.
Empiricism and tradition is based on observation by current and past herbalists. It relies on observations that a particular herbal preparation was useful with a particular diagnosis or other indication; it does not assume any particular model.
Theoretical models include that of Galen, based on humors. 
Traditional Chinese Medicine
Natural remedies, usually called herbs, a term which includes things of animal or mineral origin, are one of several means of treatment in traditional Chinese medicine (TCM). In this discussion, the focus will be on preparations of plant origins, but other sources may occasionally be used to illustrate a particular point. TCM encompasses diagnostic techniques based on a model of vitalist energy flows, using techniques including feeling the pulse at a large number of well-defined points on the body.
Herbs have four basic properties, which are interpreted within the overall body model. In the "Three Roads" model encouraged by the Chinese government, some of these properties can be at least generally related to concepts in conventional medicine.
In the late nineteenth century, attempts began to reconcile Chinese and Western concepts. It must be kept in mind that while Western medicine of that era had an excellent grasp of anatomy, it had only general principles of physiology: things, such as the gross properties of circulation that can be observed at the macroscopic level. Molecular biology was still in the future; as new systems of chemical messengers are identified, there can be interesting, if not definitive, parallels to vitalist flows. Mao Ching-i, by 1922, observed "Chinese medicine values vessel diagnosis; Western medicine values percussion and auscultation. Chinese medicine emphasize symptoms visible in one's appearance; Western medicine emphasizes dissection. Chinese medicine honors theory; Western medicine depends on practice...there are similarities, and there are differences." Western medicine looks at tubes for blood and air; Chinese medicine looks at pathways of constructive and protective influence. Both speak of the heart having an effect on blood. 
Another writer speaks of Western medicine being strongest when there can be morphological diagnosis, while Chinese medicine is better for diffuse disturbances of qi. By the 1970s, there were more scientific, but perhaps ideologically driven, attempts so synthesize. By 1976, there was less pressure to map, but to take advantages of those commonalities that could be identified. They may have overstressed attempts to map energy flows to biological mechanisms, and ignored the "demonology" that may be the TCM equivalent of psychosomatic medicine.
Further, it was observed that some traditional literature, which had not been translated, introduces concepts of medicinal "soldiers" attacking "enemies" that actually related well to the idea for drugs attacking etiologic agents, rather than the "balancing" so often mentioned in the West. Specifically, the eighteenth century writings of Hsu Ta-ch'un were mentioned. In this, he draws an analogy to Sun Tzu: These are an alternative to the vital flow based theories that could map to early Western humoral concepts. "Soldiers were introduced to eliminate violence, and, if there was no alternative, military operations must be started. Similarly, drugs were introduced to eliminate disease, and, if there is no other way, they must be employed. The principle is the same. ...it is if one were confronted by a hostile country. One takes advantage of the unilaterally marked nature of herbs and trees to attack the unilaterally dominant depots and palaces...if one checks the [enemy] at multiple places, there will be no grief over loss of life or a destroyed body."
Herbs have four main attributes:
- Nature: Most often these are called cooling or heating, but the terms moistening, relaxing and energizing are used. A cooling herb, for example, reduces indigestion. A parallel can be drawn, perhaps, to molecular agonists and antagonists, or perhaps to excitatory and inhibitory neurotransmitters.
- Taste: Basic tastes of sour, bitter, sweet or bland, spicy, and salty, and herbs are a means of categorizing, but can have further implications such as "drying", which reduce secretion.
- Affinity: This is an association for a particular organ network.
- Primary action: This describes the primary mode of the herb, which may dispel (move), astringe (restrain), purge (expel), or tonify (strengthen)."
Availability of plants
Another reason to synthesize a phytochemical is that the plant that contains it is rare. There are real-world cases where meeting the medical demand for a phytochemical could lead to extinction of the plant that produces it. The Pacific yew tree, Taxus brevifolia, was found to produce a chemical, Taxol (brand name) for paclitaxel, useful against breast cancer. Further investigation showed that it produces a phytochemical precursor for another anticancer drug, Taxotere brand of docetaxel.
The tree itself is an endangered species, and it is home to another endangered species, the spotted owl. Ethical conflicts between human, tree, and owl needs have become a popular assignment for learning debate techniques.
Methods of preparation
Obtaining yew-derived drugs is chemically complex. Other plants, such as foxglove or the opium poppy, can have strong effects in humans if the whole plant is eaten, or a simple tea is prepared from them.
Integrating herbalism with conventional medicine
Since plants do contain chemicals with significant pharmacologic effect, there is a concern with their interacting with conventionally prescribed drugs. Not infrequently, patients may not tell a conventional practitioner about their use of herbals, possibly fearing disapproval. Even if the conventional practitioner does not disapprove, there may be a lack of data about potential interactions; this does not preclude the joint use of herbal and synthetic preparations, but it does call for caution and active monitoring.
In a 2002 report, Safety of Medicines: A guide to detecting and reporting adverse drug reactions; Why health professionals need to take action', the World Health Organization observed "There are differences among countries (and even regions within countries) in the occurrence of" [adverse events], including:
the use of traditional and complementary drugs (e.g. herbal
remedies) which may pose specific toxicological problems,when used alone or in combination with other drugs.
|Hypericum||"The available evidence suggests that the hypericum extracts tested in the included trials a) are superior to placebo in patients with major depression; b) are similarly effective as standard antidepressants; c) and have fewer side effects than standard antidepressants. The association of country of origin and precision with effects sizes complicates the interpretation"|
|Pelargonium sidoides||"P. sidoides may be effective in alleviating symptoms of acute rhinosinusitis and the common cold in adults, but doubt exists"|
|Cranberry||"There is some evidence that cranberry juice may decrease the number of symptomatic UTIs over a 12 month period, particularly for women with recurrent UTIs. It's effectiveness for other groups is less certain. The large number of dropouts/withdrawals indicates that cranberry juice may not be acceptable over long periods of time. It is not clear what is the optimum dosage or method of administration (e.g. juice, tablets or capsules). Further properly designed studies with relevant outcomes are needed."|
|Ginkgo biloba||"In this study, G. biloba at 120 mg twice a day was not effective in reducing either the overall incidence rate of dementia or AD incidence in elderly individuals with normal cognition or those with MCI."|
- Select Committee appointed to consider Science and Technology, U.K. Parliament (21 November 2000), Chapter 2: Disciplines examined, Definitions of the Various CAM Therapies, Complementary and Alternative Medicine
- Anonymous (2015), Phytotherapy (English). Medical Subject Headings. U.S. National Library of Medicine.
- Matthew Wood (January 2006), - An Exploration of the Conceptual Foundations of Western Herbalism and Biomedicine With Reference to Research Design, MSc thesis, Scottish School of Herbal Medicine, University of Wales
- Charles M. Leslie, Allan Young, American Anthropological Association (1992), Asian Medical Knowledge, University of California Press, ISBN 0520073185, pp. 50-51
- Charles M. Leslie, Allan Young, American Anthropological Association (1992), Asian Medical Knowledge, University of California Press, ISBN 0520073185,Leslie, pp. 55-57
- Alternative Medical Foundation, Chinese Herbs, TRADITIONAL CHINESE MEDICINE" An Alternative and Complementary Medicine Resource Guide
- Marshall D. Murray (July-August, 1991 >), "The tree that fights cancer - Pacific yew", American Forests
- Second breast cancer drug from yew tree - Food and Drug Administration approves Taxotere, or docetaxel, September 1996
- A WebQuest Senate Debate, Taxol® and Yew
- World Health Organization (2002), Safety of Medicines: A guide to detecting and reporting adverse drug reactions; Why health professionals need to take action, WHO/EDM/QSM/2002.2, p. 9
- Linde K, Berner MM, Kriston L (2008). "St John's wort for major depression". Cochrane Database Syst Rev (4): CD000448. DOI:10.1002/14651858.CD000448.pub3. PMID 18843608. Research Blogging.
- Timmer A, Günther J, Rücker G, Motschall E, Antes G, Kern WV (2008). "Pelargonium sidoides extract for acute respiratory tract infections". Cochrane Database Syst Rev (3): CD006323. DOI:10.1002/14651858.CD006323.pub2. PMID 18646148. Research Blogging.
- Anonymous (2015), EPs 7630 (English). Medical Subject Headings. U.S. National Library of Medicine.
- Jepson RG, Craig JC (2008). "Cranberries for preventing urinary tract infections". Cochrane Database Syst Rev (1): CD001321. DOI:10.1002/14651858.CD001321.pub4. PMID 18253990. Research Blogging.
- DeKosky ST, Williamson JD, Fitzpatrick AL, et al (November 2008). "Ginkgo biloba for prevention of dementia: a randomized controlled trial". JAMA 300 (19): 2253–62. DOI:10.1001/jama.2008.683. PMID 19017911. Research Blogging.