Ascorbic acid is therefore essential for a healthy body. But:
Does the vitamin have a preventive or even healing effect on diseases?
For some time now, there has been an unconfirmed belief that vitamin C can be used against cancer.
Particularly in the USA, highly concentrated intravenous vitamin C has already been advertised by various suppliers.
The only approved treatment methods for tumor diseases are operation, chemotherapy and radiation therapy. Suppliers of the vitamin C "remedy" put patients at risk by promising what they may not be able to provide. There is no clinical data to confirm effective treatment with high levels of the vitamin.
However, it is interesting to assess the thoughts behind this claim systematically.
Because although it should not be used for treatment, vitamin C still has proven beneficial for our health.
So where does one draw the line and what has actually been studied?
Background of the treatment with vitamin C
The preventive effect of vitamin C on chronic diseases can be attributed to its antioxidant properties and the support of a healthy immune system.
It is also suspected that vitamin C may have an antiproliferative effect on cancer cells in very high concentrations4. This means that vitamin C influences the ability of tumor cells to multiply.
It has also been observed that very high doses of vitamin C can make cancer cells more sensitive to certain cytostatic agents5. Both of these points affect the cell division of cancer cells and might slow down growth. Which hasn't been proven in humans.
Chemotherapies have massive side effects that weaken the patient because they are designed to combat fast-dividing cells, which include cancer cells as well as cells of the immune system that are also destroyed by the therapy.
In order to curb the side effects of chemotherapy, vitamin C was tested as a complementary therapeutic and was able to reduce the toxicity of some chemotherapeutics in animal experiments, but above all to support the immune system6,7.
Chemotherapy and vitamin C deficiency
In general, tumor patients easily get a vitamin C deficiency due to their altered metabolism8. The low serum concentration of the vitamin in cancer patients, despite adherence to the recommended daily dose, could be caused primarily by the increased need.
During cancer treatment, there is a massively increased consumption of the antioxidant for the detoxification of reactive oxygen radicals, which are formed during surgery, chemo, and radiation.
Oxidative stress and wound healing thus consume a great deal of vitamin C to keep the body in balance despite the high stress9,10,11.
Why the FDA banned intravenous vitamin C in America
The FDA is the US Food and Drug Administration under its Department of Health. At the end of 2010, this agency sent a reminder to the Californian company McGuff, which at that time manufactured intravenous vitamin C preparations, demanding that production be discontinued. At first, there were massive outcries.
So what happened?
The FDA is investigating whether drugs are marketed without their prior approval. To be approved there needs to be an examination of safety and efficacy.
In contrast to conventional vitamin C preparations, which are bought and used to compensate for deficiencies or to support health in stressful situations, the intravenous variant is a drug. It's marketed as remedy. But this use is not confirmed.
The verdict: the preparations, which are highly concentrated, are not designed for diffuse health care, but "were meant to treat a disease which should neither be self-diagnosed nor treated without a therapist "12.
The FDA does not rule out efficacy forever. It merely protects patients from an unconfirmed treatment method that may be ineffective or even pose additional risks.
 Ang, Abel et al. “Vitamin C and immune cell function in inflammation and cancer” Biochemical Society transactions vol. 46,5 (2018): 1147-1159.
 Wintergerst ES, Maggini S, Hornig DH (2006). "Immune-enhancing role of vitamin C and zinc and effect on clinical conditions". Annals of Nutrition & Metabolism. 50 (2): 85–94. doi:10.1159/000090495.
 EFSA Panel on Dietetic Products, Nutrition and Allergies (2015). "Vitamin C and contribution to the normal function of the immune system: evaluation of a health claim pursuant to Article 14 of Regulation (EC) No 1924/2006". EFSA Journal. 13 (11): 4298. doi:10.2903/j.efsa.2015.4298.
 Fromberg A, Gutsch D, Schulze D, Vollbracht C, Weiss G, Czubayko F, Aigner A, Ascorbate exerts anti-proliferative effects through cell cycle inhibition and sensitizes tumor cells towards cytostatic drugs. Cancer Chemother Pharmacol 67(5): 1157-1166, 2010.
 Kurbacher CM, Wagner U, Kolster B, Andreotti PE, Krebs D, Bruckner HW, Ascorbic acid (vitamin C) improves the antineoplastic activity of doxorubicin, cisplatin, and paclitaxel in human breast carcinoma cells in vitro. Cancer Lett 103(2):183-189, 1996.
 El-Merzabani MM, El-Aaser AA, Osman AM, Ismael N, Abu el-Ela F, Potentiation of therapeutic effect of methanesulphonate and protection against its organ cytotoxicity by vitamin C in Ehrlich ascites carcinoma bearing mice. J Pharm Belg 44(2): 109-116, 1989.
 Prasad SB, Giri A, Arjun J, Use of subtherapeutical dose of cisplatin and vitamin C against murine Dalton's lymphoma. Pol J Pharmacol Pharm 44(4):383-391, 1992.
 Vollbracht, C., Schneider, B., Leendert, V., Weiss, G., Uerbach, L., Beuth, J., Beuth, J., Intravenous Vitamin C Administration Improves Quality of Life in Breast Cancer Patients during Chemo-/Radiotherapy and Aftercare: Results of a Retrospective, Multicentre, Epidemiological Cohort Study in Germany, In Vivo, 11-12/2011, vol. 25, no. 6, 983-990.
 Beuth JH N, van Leendert R, Basten R, Noehle M, Schneider B, Safety and efficacy of local administration of contractubex to hypertrophic scars in comparison to corticoid treatment. Results of a multicenter, comparative epidemiological cohort study in Germany. In Vivo 20: 277-284, 2006.
 Akinloye O, Adaramoye O, Kareem O, Changes in antioxidant status and lipid peroxidation in Nigerian patients with prostate carcinoma. Pol Arch Med Wewn 119(9): 526-532, 2009.
 Esme H, Cemek M, Sezer M, Saglam H, Demir A, Melek H, Unlu M, High levels of oxidative stress in patients with advanced lung cancer. Respirology 13(1):112-116, 2008.
 Warning Letter an McGuff Compounding Pharmacy Services Inc.: https://www.fda.gov/ICECI/EnforcementActions/WarningLetters/2015/ucm473759.htm.
 Hathcock JN, Azzi A, Blumberg J, Bray T, Dickinson A, Frei B, Jialal I, Johnston CS, Kelly FJ, Kraemer K, Packer L, Parthasarathy S, Sies H, Traber MG. Vitamins E and C are safe across a broad range of intakes. Am J Clin Nutr. 2005 Apr;81(4):736-45.