INTRODUCTION — Homeopathy is a system of complementary and integrative medicine based upon the principle of "similars": the most appropriate treatment for a patient is a substance that, when given to a healthy person, produces symptoms similar to those that the patient is experiencing. Homeopathic preparations are created through a process known as "potentization," in which the starting material is serially diluted and "succussed" (shaken). Homeopathic medicines include minerals, plants, or animal substances that are pulverized, mixed with a water-alcohol solution, and then potentized multiple times, often well past the point at which any of the original substance theoretically remains. The resulting solution is most commonly applied to a sucrose pill and administered sublingually.

Homeopathy and homeopathic medicines are distinct from herbal and dietary supplements; these are discussed separately. (See "Overview of herbal medicine and dietary supplements" and "Complementary and alternative medicine in pediatrics" and "Complementary and alternative therapies for rheumatic disorders" and "Complementary and alternative therapies for cancer".)

EPIDEMIOLOGY OF HOMEOPATHIC PRACTICE — Homeopathy is more widely used in Europe and Asia than in the United States, where it is not widely practiced [1-3]. In a 2012 National Health Interview Survey (NHIS) evaluating the use of complementary and integrative medicine in the United States, 2.2 percent of adults and 1.8 percent of children ages 4 to 17 years had used homeopathic medicines within the past year [4,5]. Most individuals using these medicines had self-treated with over-the-counter products and did not see a homeopathic provider for guidance [6]. However, among those taking homeopathic medicines, those who saw a homeopath were more likely to find the treatment helpful than those who did not. The most common categories of problems treated with homeopathic medicines included respiratory and otolaryngological (18.5 percent), musculoskeletal (12.3 percent), fatigue, sleep problems, stress or chronic pain (7.7 percent), gastrointestinal (5.0 percent), neurologic (3.4 percent), and mental health (2.1 percent).

FOUNDATIONS OF HOMEOPATHY

Principle of similars — Homeopathy was created at the end of the 18^th century by the German physician Samuel Hahnemann after noting that ingestion of the bark of Cinchona officinalis (later found to contain quinine), which was used for the treatment of malaria, caused symptoms similar to malaria [7]. It was this experience that led Hahnemann to conclude "similia similibus curantur" or "let like be cured by like."

Although the principle of similars is not unique to homeopathy (precursors are found in the Hippocratic corpus and writings of Paracelsus), Hahnemann was the first to integrate this principle into an entire system of therapeutics.

Homeopathic pathogenetic trials — Based upon Hahnemann's observations, experiments (or "provings") were performed with human volunteers, comparing the effects of substances with placebos. In fact, early homeopaths conducted some of the first placebo-controlled trials [8]. Subjects in such trials keep a detailed diary of every sensation, feeling, mood change, and physical change that occurs for several days to weeks after ingesting a substance. In homeopathy, symptoms are generally recorded in much more nuanced detail than in conventional medicine.

Creation of homeopathic medicines — Homeopathic medicines are manufactured according to a process known as potentization (also referred to as "attenuation" or "dynamization"). Many of the conventional medicines of Hahnemann's time (eg, arsenic, mercury, belladonna) were highly toxic, and initial provings were done with undiluted substances. He experimented with attenuating the dose of homeopathic preparations through serial dilutions and succussions; this process not only reduced the adverse effects but, according to his observations, paradoxically seemed to render the preparations more potent.

Most modern homeopathic medicines begin with a mineral, plant, or animal substance that is pulverized and mixed with a water-alcohol solution or triturated in lactose [9]. Many of these substances are highly toxic in their initial concentrations; the initial preparation undergoes potentization, consisting of serial dilutions and succussions.

The dose of a homeopathic medicine is indicated by a combined number-letter designation, with the number representing the total number of potentization steps and the letter representing the degree of dilution at each step (eg, the roman numeral "X" or a "D" for 1:10 dilutions, and the roman numeral "C" for 1:100 dilutions) [7]. As examples:

●A homeopathic preparation with a 6X potency has undergone 6 serial potentizations with a 1:10 dilution at each step; the final concentration is 10^-6 from the starting material.

●A homeopathic preparation with a 30C potency has undergone 30 serial potentizations with a 1:100 dilution at each step; the final concentration is 10^-60 from the starting material.

Homeopathic medicines are commonly diluted from 10^-6- to 10^-10,000-fold; medicines diluted beyond 10^-23 are sometimes called ultra-high dilutions, ultramolecular dilutions, or ultra-low doses. Once potentized, most medicines are applied to small lactose or sucrose pellets, which are taken sublingually. Homeopathic medicines can also be prepared and administered in injectable, liquid (for oral use), or topical forms.

Homeopathic medicines are manufactured by dedicated pharmaceutical companies following specific criteria outlined in the Homeopathic Pharmacopoeia of the United States (HPUS) or similar documents [10-12].

Homeopathic reference materials — Homeopathic practitioners consult reference material to determine the most appropriate medicine to treat a specific symptom or constellation of symptoms. Books, collectively known as "materia medica," contain information on available medicines including descriptions of symptoms that they both elicit and treat, as well as toxicological information on the foundation substances. A repertory is a reference book in which symptoms, listed alphabetically and grouped by organ systems and anatomic location, are followed by a list of homeopathic medicines that can treat the symptom. In addition, medicines are typically ranked according to the degree that they produce the symptom.

Historically, the process of matching symptoms to medicines was tedious, performed by cross-referencing between different books; computer software has greatly aided this process.

Homeopathic approach to selecting a medicine — The homeopathic practitioner elicits a detailed description of a patient's symptoms, then consults a repertory to identify the medicine that most closely matches the totality of symptoms. The process is repeated for all of the patient’s symptoms (or in some cases, the most characteristic symptoms), creating a list that can be further narrowed by consulting a materia medica.

This process leads to individualized treatment of the unique manifestations of an illness in a particular patient. For this reason, two patients with the same disease process, but with slightly different symptoms, may be prescribed different homeopathic medicines. As an example, a patient with a viral upper respiratory tract infection who presents with a thin, clear nasal discharge with irritation and stinging of the eyes may be treated with a preparation made from onion (Allium cepa), while another patient with the same infection who has thick, yellow nasal discharge with loss of thirst may be treated with Pulsatilla, a preparation made from the pasque flower [13].

Styles of contemporary homeopathy — There are several different styles of homeopathy that are commonly practiced today [7], including classical, clinical, and complex homeopathy.

●Classical homeopathy – Classical homeopathy involves the method described above, in which the totality of the patient’s symptoms is taken into account to find the most appropriate homeopathic medicine. Generally, only one medicine is prescribed at a time. There are a variety of different schools of classical homeopathy that follow variations on this approach.

●Clinical homeopathy – Clinical homeopathy generally focuses on a narrower subset of symptoms, using a more pathophysiologic approach and treating with lower potency (less dilute, usually no higher than 30C) medicines. In clinical homeopathy, multiple homeopathic medicines may be prescribed at once, each medicine for a specific indication.

●Complex homeopathy – Complex homeopathy uses multiple homeopathic medicines combined into a single compound to target a symptom complex (eg, allergic rhinitis) in an attempt to cover the natural variation in symptoms for a large majority of patients. Such preparations are typically low potency (less dilute) and are widely available over the counter.

LIMITED EVIDENCE SUPPORTING THE USE OF HOMEOPATHY

Clinical outcomes — Data supporting the use of homeopathic medicines for clinical conditions are limited.

Meta-analyses of randomized trials of homeopathic treatment are limited by the quality of available trials [14]. As examples, in a 2017 meta-analysis including 54 trials of non-individualized homeopathic treatments for a wide variety of conditions, only three included trials were judged to be high-quality with low risk of bias [15]. Similarly, in a 2014 meta-analysis including 32 trials of individualized homeopathic treatments for various conditions, only three included studies that were considered reliable [16].

High-quality data supporting the superiority of homeopathic treatment over placebo are lacking. Although several meta-analyses reported an overall benefit of homeopathic medicines in general (rather than for a specific medical condition), most of the included trials were deemed low-quality evidence [17-19]. No significant effect was found when only the highest-quality trials were analyzed [19].

However, homeopathic medicines may possibly be useful for the symptomatic management of certain conditions, particularly upper respiratory tract infections (eg, one study showed that practitioners who provided homeopathic treatments were less likely to also prescribe antibiotics for upper respiratory tract infections) [20,21], fibromyalgia [22], endometriosis-associated pelvic pain [23], general climacteric symptoms in peri/post-menopausal women, and postmenopausal hot flashes [24,25].

Systematic reviews of randomized trials evaluating homeopathy for other specific conditions failed to show evidence of benefit (eg, attention deficit/hyperactivity disorder) or concluded that there was insufficient evidence to determine whether homeopathy was effective (eg, asthma, dementia, and irritable bowel syndrome) [26-29].

Controversies — In addition to the lack of high-quality data supporting the overall efficacy of homeopathy, the assertion that the medicines are rendered more potent with increased numbers of serial dilutions and succussions is another reason that homeopathy remains contentious; homeopathy seems to “defy logic” and the biomedical paradigm [30].

However, practitioners of homeopathy identify biochemical, immunological, botanical, cell biological, and zoological experiments that support the foundational principles of homeopathy [9,31-42]. As an example, materials science research has demonstrated the presence of source nanoparticles in homeopathic preparations [35,36,39]; these may modulate the stress response network and have possible hormetic effects [9,40-42].

Safety — Treatment with homeopathic medicines, particularly oral and topical formulations, is generally safe. As an example, in a meta-analysis including 41 trials and over 6000 patients, homeopathic treatment had no more side effects than placebo (odds ratio [OR] 1.03, 95% CI 0.89-1.20) [43]. Furthermore, in a German study evaluating post-marketing adverse drug effects of injectable homeopathic preparations, there were less than four reactions per one million ampules sold, with at least a quarter of these being a local injection site reactions [44].

However, since 2009, the US Food and Drug Administration (FDA) has recalled several over-the-counter homeopathic products due to mislabeling. In one case, the product was mislabeled as homeopathic but actually contained sizeable amounts of zinc, which led to anosmia in some individuals [45]. In another case, a product was marketed for asthma symptoms, a non-over-the-counter indication that is not permitted [46]. In 2017, the FDA recalled homeopathic teething tablets, some of which contained atropine and scopolamine in excess of the labeled quantities [47]. However, of the over 400 possible patient incident reports filed with the FDA, the majority were requests for information and not reports of harm [48].

REGULATION OF HOMEOPATHY

Regulation of homeopathic medicines — The regulation of homeopathy and homeopathic medicines varies widely throughout the world [49]. In the United States, the Homeopathic Pharmacopoeia Convention of the United States is responsible for maintaining the Homeopathic Pharmacopeia of the United States (HPUS), the guide for manufacturing homeopathic medicines [50,51]. Prior to 2017, the US Food and Drug Administration (FDA) recognized the HPUS, with minimal regulatory interference in the marketing of homeopathic medicines. In 2017, the FDA issued draft guidance recommending more rigorous regulation of homeopathic medicines [52]; that draft guidance was revised in 2019 [53]. The FDA has recommended a "risk-based" enforcement approach for the marketing of certain homeopathic preparations, such as those that purport to treat more serious health conditions (eg, asthma), those that are intended for use in vulnerable populations (eg, infants and children, older adults, and pregnant women), and those that are intended for administration other than an oral or topical route.

There are no homeopathic preparations or products marketed in the United States that are FDA approved.

Certification of homeopathic practitioners — There are several homeopathic certification boards in North America, and many of these organizations offer publicly available lists of certified practitioners. The American Board of Homeotherapeutics offers the Diplomate of Homeotherapeutics (D.Ht.) certificate to MDs or DOs [54], and the Homeopathic Academy of Naturopathic Physicians offers the Diplomate of the Homeopathic Academy of Naturopathic Physicians (DHANP) certificate to naturopaths in North America [55].

The Council for Homeopathic Certification (CHC) offers the Certified Classical Homeopath (CCH) designation to lay practitioners [56]. Practitioners certified by the CHC can also become members of the North American Society of Homeopaths (NASH) and may use the designation "RSHom (NA)" after their names [57].

SUMMARY

●Homeopathy is a system of complementary and integrative medicine based upon the principle of "similars": the most appropriate treatment for a patient is a substance that, when given to a healthy person, produces symptoms similar to those that the patient is experiencing. Homeopathic preparations are created through a process known as "potentization," in which the starting material is serially diluted and "succussed" (shaken). Homeopathic preparations are potentized multiple times, often well past the point at which any of the original substance theoretically remains. (See 'Introduction' above.)

●The dose of a homeopathic medicine is indicated by a combined number-letter designation, with the number representing the total number of potentization steps and the letter representing the degree of dilution at each step. Homeopathic medicines are commonly diluted from 10^-6- to 10^-10,000-fold; medicines diluted beyond 10^-23 are sometimes called ultra-high dilutions, ultramolecular dilutions, or ultra-low doses. (See 'Creation of homeopathic medicines' above.)

●There are several different styles of homeopathy that are commonly practiced today, including classical, clinical, and complex homeopathy. (See 'Styles of contemporary homeopathy' above.)

●Data supporting the use of homeopathic medicines for clinical conditions are limited. (See 'Clinical outcomes' above.)

●Treatment with homeopathic medicines, particularly oral and topical formulations, is generally safe. (See 'Safety' above.)

●The US Food and Drug Administration (FDA) has recommended a "risk-based" enforcement approach for the marketing of certain homeopathic preparations, such as those that purport to treat more serious health conditions (eg, asthma), those that are intended for use in vulnerable populations (eg, infants and children, older adults, and pregnant women), and those that are intended for administration other than an oral or topical route. However, there are no homeopathic preparations or products marketed in the United States that are FDA approved. (See 'Regulation of homeopathic medicines' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Kimball Atwood, MD, who contributed to an earlier version of this topic review.