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The therapy has not been independently tested or subjected to randomized controlled trials, and is illegal to market in the United States as a cancer cure as it lacks proven effectiveness. The Gerson Institute points to observational studies and case reports collected by Gerson himself as anecdotal evidence of the efficacy of the treatment. In his book, Gerson cites the "Results of 50 Cases"; however, the U.S. National Cancer Institute reviewed these 50 cases and was unable to find any evidence that Gerson's claims were accurate. Several retrospective case series have been published in the alternative medical literature; however, these have suffered from signficant methodological flaws.

The American Cancer Society reports that "[t]here is no reliable scientific evidence that Gerson therapy is effective in treating cancer, and the principles behind it are not widely accepted by the medical community. It is not approved for use in the United States." In 1947, the National Cancer Institute reviewed 10 "cures" submitted by Gerson; however, all of the patients were receiving standard anticancer treatment simultaneously, making it impossible to determine what effect, if any, was due to Gerson's therapy. A review of the Gerson Therapy by Memorial Sloan-Kettering Cancer Center concluded: "If proponents of such therapies wish them to be evaluated scientifically and considered valid adjuvant treatments, they must provide extensive records (more than simple survival rates) and conduct controlled, prospective studies as evidence."

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Pseudoscience is defined as a body of knowledge, methodology, belief, or practice that is claimed to be scientific or made to appear scientific, but does not adhere to the scientific method, lacks supporting evidence or plausibility, or otherwise lacks scientific status.
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Pseudosciences have been characterised by the use of vague, exaggerated or untestable claims, over-reliance on confirmation rather than refutation, lack of openness to testing by other experts, and a lack of progress in theory development. [...Also] assertion of claims of a conspiracy on the part of the scientific community to suppress the results.

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