Vitamin C, oral
Also called: ascorbic acid, ascorbate by mouth.
Last reviewed 2026-09-16. How this works.
Treating cancer Grade D
No human evidence of benefit, or none that survived testing.
Taken by mouth, vitamin C cannot reach the levels the lab work needs, and two trials found it did not help.
Two randomised trials found nothing, and the oral ceiling explains why.
The claim
The claim is that high-dose vitamin C treats cancer. It is among the oldest claims in the book and it produced one of its most instructive stories.
What it does
The mechanism proposed is that at very high concentrations ascorbate acts as a source of hydrogen peroxide around cells rather than as an antioxidant. Whether that happens depends entirely on the concentration reached, and this is where the routes separate.4
Taken by mouth, the plasma concentration is held at a ceiling of roughly 100 to 220 micromolar by absorption, tissue uptake and the kidney. Taking more does not push it higher. That ceiling is a property of the body, not of the preparation.4
The evidence
The story starts with a Scottish series reporting that patients given ascorbate survived far longer than matched controls, a mean of more than four times as long, with a follow-up paper reporting a larger difference still.23
Three features of those studies matter more than the numbers. Controls were selected retrospectively from records rather than randomised. Nobody was blinded, and the clinician who chose to give the vitamin also judged when each patient became untreatable, which is the moment the survival clock started. And the later paper was not an independent replication, its authors stating the two sets of patients were in part the same.32
Credit where it is due: they addressed one criticism directly by also measuring survival from the known date of first hospital attendance.3
Then it was tested properly. A randomised double-blind trial in 100 people with advanced bowel cancer, none of whom had received previous chemotherapy and all in good general condition, found no advantage over placebo. That trial was built specifically to answer the published objection to an earlier one, and it got the same answer.1
The sequence is the point. An objection was raised, the trialists accepted it, rebuilt the study to answer it, and the result did not change.1
One caution from within the field: the original Scottish population was at substantial risk of genuine vitamin C deficiency, so part of any benefit may have been correction of that rather than an anti-cancer effect. That is the baseline-status principle again, for the third time in this chapter.4
The studies this rests on
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High-dose oral vitamin C versus placebo in advanced cancer without prior chemotherapy1
A randomised trial, double blind, measured a clinical outcome, 100 participants.
no advantage over placebo.
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Cameron and Pauling ascorbate series23
A series of cases, with no comparison group, observational, open label, measured survival, 100 participants.
longer survival reported against retrospectively selected controls, unblinded, with the treating clinician judging the start of the survival clock.
What is absent
Nothing in this record shows that vitamin C taken by mouth changes what a cancer does. The route cannot reach the concentration the proposed mechanism requires, and the trials that tested it found nothing.
Scope: the oral route, any tumour type.
Established by trial scope, checked 2026-09-16. Result: the randomised trial in this record found no advantage over placebo, and the measured oral ceiling is far below the concentrations the mechanism requires.14
What to ask
Is there any reason taking vitamin C by mouth would affect my treatment?
The dish-to-person gap
This is the entry where a measured concentration decides the question. Taken by mouth, vitamin C reaches a plasma concentration of roughly 100 to 220 micromolar and no higher, because intestinal absorption, tissue uptake and the kidney together hold it there. Taking more does not raise it.4
When to call your team — no warning signs are recorded
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
The register records no warning signs for oral vitamin C. That is not a statement that nothing can go wrong.
Interactions
One interaction is recorded elsewhere in this register rather than here: the Cochrane review of laetrile records that the risk of cyanide poisoning could increase alongside vitamin C. That is on the apricot kernels page and it belongs on this one too.
Apricot kernels or laetrile, with vitamin C
Recorded as documented, between two compounds rather than against a cancer treatment.
The Cochrane review of laetrile records that the risk of cyanide poisoning could increase alongside vitamin C, and in people who are vegetarian with low vitamin B12. Vitamin C is among the most commonly taken supplements in this readership, which is why this is recorded on both pages. It runs between two compounds rather than against a cancer treatment, so it never appears in the treatment-class list.
Can you look at what I have read about laetrile and vitamin C together raising the risk of cyanide poisoning?
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
The oral route has been tested twice in randomised trials. The open question is about the intravenous route, which has its own entry.
References
Moertel CG, Fleming TR, Creagan ET, Rubin J, O'Connell MJ, Ames MM. High-dose vitamin C versus placebo in the treatment of patients with advanced cancer who have had no prior chemotherapy: a randomized double-blind comparison. New England Journal of Medicine 1985;312(3):137-141.
doi:10.1056/NEJM198501173120301 · PubMed 3880867
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 16.
Cameron E, Campbell A. The orthomolecular treatment of cancer. II. Clinical trial of high-dose ascorbic acid supplements in advanced human cancer. Chemico-Biological Interactions 1974;9(4):285-315. Cameron E, Pauling L. Supplemental ascorbate in the supportive treatment of cancer: prolongation of survival times in terminal human cancer. PNAS 1976;73(10):3685-3689.
doi:10.1016/0009-2797(74)90019-2
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 8.
Cameron E, Pauling L. Supplemental ascorbate in the supportive treatment of cancer: reevaluation of prolongation of survival times in terminal human cancer. PNAS 1978;75(9):4538-4542.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 9.
Padayatty SJ, Levine M, and colleagues, 2004, and related pharmacokinetic work.
doi:10.7326/0003-4819-140-7-200404060-00010 · PubMed 15068981
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 10. The author string is abbreviated in the source that was read and has not been expanded.