Functional medicine is a form of alternative medicine which proponents say focuses on interactions between the environment and the gastrointestinal, endocrine, and immune systems. Practitioners attempt to develop individual treatment plans for people they treat. Functional medicine encompasses a number of unproven and disproven methods and treatments, and has been criticized for being pseudoscientific.
The discipline of functional medicine is vaguely defined by its proponents. Oncologist David Gorski has written that the vagueness is a deliberate tactic which facilitates the discipline's promotion, but that in general it centers around unnecessary and expensive testing procedures performed in the name of "holistic" health care.
Functional medicine significantly departs from mainstream medicine in its emphasis on treatments and concepts of health and disease which are not currently known to be effective or which have been shown to be ineffective by clinical research. These include
- Orthomolecular medicine 
- Detoxification of undocumented toxins.
- "Biochemical Individuality" (i.e. the notion that the nutritional needs, chemical constitution and disease states are unique for every individual; this represents a recrudescence of the mainstream medical conception of disease common before the development of germ theory)
- Organ reserve
- Diagnosis of chronic occult infections (e.g. so-called chronic Lyme disease)
- Homeopathy, including "Biopuncture", the injection of homeopathic remedies
- Dubious nutritional interventions, including avoidance of gluten for people who do not have celiac disease or non-celiac gluten sensitivity
- Antivaccine advocacy including promotion of the discredited link between MMR vaccine and autism (the retracted Lancet paper by Wakefield et al. is cited in The Textbook of Functional Medicine)
- Leaky gut syndrome
Jeffrey Bland and Susan Bland founded the Institute for Functional Medicine in 1991 as a division of HealthComm. That year, the U.S. Federal Trade Commission said that Jeffrey Bland's corporations HealthComm and Nu-Day Enterprises had falsely advanced claims that their products could alter metabolism and induce weight loss. The FTC found that Bland and his companies violated that consent order in 1995 by making more exaggerated claims. The UltraClear dietary program was said to provide relief from gastrointestinal problems, inflammatory and immunologic problems, fatigue, food allergies, mercury exposure, kidney disorders, and rheumatoid arthritis. The companies were forced to pay a $45,000 civil penalty.
The opening of centers for functional medicine at the Cleveland Clinic Foundation and at George Washington University has been cited by Gorski as a depressing example of how pseudoscientific quackery is infiltrating medical academia.
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