The use of intravenous, high-dose vitamin C in cancer therapy has remained controversial for a long time. Despite this, it has been commonly administered for decades by complementary health care practitioners. This article reviews the existing clinical and pre-clinical data on the efficacy and safety of L-ascorbic acid in oncological diseases, with the emphasis on the following issues: 1) Does vitamin C demonstrate therapeutic efficacy? In which clinical situations? 2) What is the mechanism of vitamin C action? 3) Is the use of vitamin C in oncology safe? 4) What is the effect of vitamin C on other therapies used simultaneously? Based on the analysis of available literature, it has been concluded that although L-ascorbic acid exhibits a possible anticancer effect, as shown mainly by preclinical studies, the reliable clinical data justifying the introduction of vitamin C into conventional oncology has been lacking so far. In fact, the following issues still remain unknown: types of cancer that could be sensitive to high-dose vitamin C therapy, groups of patients that could benefit from such management, doses, routes of administration and pharmaceutical forms that could provide an optimal anticancer effect, the molecular mechanism of vitamin C anticancer action, the impact of vitamin C on the concurrently used therapies, including chemotherapy, impact of vitamin C on the quality of life of oncological patients. So far, the available clinical trials have not confirmed the positive reports from experimental studies. Therefore, it is suggested that patients intending to undrgo such treatment should be informed that there is no evidence to justify the anticancer efficacy of vitamin C infusions and that in certain populations (i.e. patients with deficiency of glucose 6-phosphate dehydrogenase, renal dysfunction or haemochromatosis) vitamin C infusions may cause serious adverse effects. However, there seems to be sufficient evidence to continue research on the role of vitamin C in cancer treatment.
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