Gerson's therapy has not been independently tested or subjected to randomized controlled trials, and thus is illegal to market in the United States. The Gerson Institute promotes the therapy by citing patient testimonials and other anecdotal evidence. Gerson published a book discussing the alleged success of the therapy in 50 patients, but a review by the U.S. National Cancer Institute was unable to find any evidence that Gerson's claims were accurate. The NCI found that no in vivo animal studies had been conducted. Similarly, case series by Gerson Institute staff published in the alternative medical literature suffered from methodological flaws, and no independent entity has been able to reproduce the claims.
Attempts to independently check the results of the therapy have been negative. A group of 13 patients sickened by elements of the Gerson Therapy were evaluated in hospitals in San Diego in the early 1980s; all 13 were found to still have active cancer. An investigation by Quackwatch found that the institute's claims of cure were based not on actual documentation of survival, but on "a combination of the doctor's estimate that the departing patient has a 'reasonable chance of surviving', plus feelings that the Institute staff have about the status of people who call in".
A 1994 article in the Journal of Naturopathic Medicine attempted to follow 39 Gerson patients in Tijuana. Patient interviews were used to confirm the existence and stage of cancer; most patients were unaware of the stage of their tumor, and medical records were not available. Most patients were lost to follow-up; of the patients successfully followed, 10 died and six were alive at their last follow-up. Review of this study pointed out its "obvious flaws", including "the majority of patients lost to follow-up, lack of access to detailed medical records, and reliance upon patients for disease stage information"; the authors themselves regarded the results as unclear.
The American Cancer Society reported that "there 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 claimed 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". In 1959, the National Cancer Institute (NCI) again reviewed cases of patients treated by Gerson. The NCI found that the available information did not prove the regimen had benefit. Cancer Research UK states that "Available scientific evidence does not support any claims that Gerson therapy can treat cancer. Gerson therapy can be very harmful to your health."
