Alternative medicine, fringe medicine, pseudomedicine or simply questionable medicine is promotion or use of practices which are unproven, disproven, impossible to prove, or excessively harmful in relation to their effect — in the attempt to achieve the healing effects of medicine. It differs from experimental medicine in that the latter employs responsible investigation, and is discarded when shown ineffective. The scientific consensus is that alternative therapies either do not, or cannot, work. In some cases laws of nature are violated by their basic claims; in others the treatment is so much worse that its use is unethical. Alternative practices, products, and therapies range from those which are simply ineffective to those having known harmful and toxic effects.
Alternative therapies may be credited for perceived improvement through placebo effects, decreased use or effect of medical treatment (and therefore either decreased side effects; or nocebo effects towards standard treatment), or the natural course of the condition or disease. Alternative treatment is not the same as experimental treatment or traditional medicine, although both can be misused in ways that are alternative. Alternative or complementary medicine is dangerous because it may discourage people from getting the best possible treatment, and may lead to a false understanding of the body and of science.
Alternative medicine is used by a significant number of people, though its popularity is often overstated. Large amounts of funding go to testing alternative medicine, with more than US$2.5 billion spent by the United States government alone. Almost none show any effect beyond that of false treatment, and most studies showing any effect have been statistical flukes. Alternative medicine is a highly profitable industry, with a strong lobby. This fact is often overlooked by media or intentionally kept hidden, with alternative practice being portrayed positively when compared to "big pharma". The lobby has successfully pushed for alternative therapies to be subject to far less regulation than conventional medicine. Alternative therapies may even be allowed to promote use when there is demonstrably no effect, only a tradition of use. Regulation and licensing of alternative medicine and health care providers varies between and within countries. Despite laws making it illegal to market or promote alternative therapies for use in cancer treatment, many practitioners promote them. Alternative medicine is criticized for taking advantage of the weakest members of society.
Terminology has shifted over time, reflecting the preferred branding of practitioners. For example, the United States National Institutes of Health department studying alternative medicine, currently named National Center for Complementary and Integrative Health, was established as the Office of Alternative Medicine and was renamed the National Center for Complementary and Alternative Medicine before obtaining its current name. Therapies are often framed as "natural" or "holistic", in apparent opposition to conventional medicine which is "artificial" and "narrow in scope", statements which are intentionally misleading. When used together with functional medical treatment, alternative therapies do not "complement" (improve the effect of, or mitigate the side effects of) treatment. Significant drug interactions caused by alternative therapies may instead negatively impact functional treatment by making prescription drugs less effective, such as interference by herbal preparations with warfarin.

Alternative diagnoses and treatments are not part of medicine, or of science-based curricula in medical schools, nor are they used in any practice based on scientific knowledge or experience. Alternative therapies are often based on religious belief, tradition, superstition, belief in supernatural energies, pseudoscience, errors in reasoning, propaganda, fraud, or lies. Alternative medicine is based on misleading statements, quackery, pseudoscience, antiscience, fraud, and poor scientific methodology. Promoting alternative medicine has been called dangerous and unethical. Testing alternative medicine that has no scientific basis has been called a waste of scarce research resources. Critics state that "there is really no such thing as alternative medicine, just medicine that works and medicine that doesn't", that the very idea of "alternative" treatments is paradoxical, as any treatment proven to work is by definition "medicine".