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Home » Vitamin C does not prevent or treat cancer — let’s end this myth

Vitamin C does not prevent or treat cancer — let’s end this myth

One of the frequently made claims from the alternative medicine world is that vitamin C prevents cancer. Or cures cancer. But is there any real science behind vitamin C and cancer?

Of course, there are over hundreds of different cancers, each with a different etiology, pathophysiology, and prognosis, so it’s rather difficult to believe that vitamin C has that much effect on any of those cancers. But the claims, and their adherents, persist despite the lack of robust evidence supporting these claims.

Frankly, there are just a handful of ways to prevent cancer. One of those ways, eating a balanced diet, implies consuming appropriate amounts of nutrients, like vitamin C, I suppose. But does it mean that taking a handful of vitamin C tablets has some beneficial effect on cancer prevention or treatment? Well, let’s take a look.

Photo by Bruna Branco on Unsplash

What is vitamin C?

Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables. It is also sold as a dietary supplement with many unsupported claims associated with its use as a supplement.

The major use, in humans, for vitamin C is for the prevention and treatment of scurvy.

Vitamin C is an essential nutrient involved in the repair of tissue, the formation of collagen, and the enzymatic production of certain neurotransmitters. It is required for the functioning of several enzymes and is important for immune system function. It also functions as an antioxidant.

Because many people misinterpret what is written here, vitamin C is necessary for normal immune system functioning. It does not mean that more vitamin C is going to boost your immune system into some all-powerful function that destroys any disease that comes your way. The best we can do with our immune system is to keep it running well — we cannot boost it.

Most animals can synthesize their own vitamin C so they do not require it from their food. However, apes (including humans) and monkeys (although non-ape, non-monkey primates synthesize their own), most bats, some rodents, and certain other animals must acquire it from dietary sources.

Vitamin C and cancer – Linus Pauling

If there’s anyone responsible for the whole vitamin C and various diseases fad, it has got to be Linus Pauling, a two-time Nobel Prize winner (an extremely rare honor). By the 1960s, he had been convinced that massive doses of vitamins, a field now called orthomolecular medicine, considered alternative medicine, could prevent a lot of diseases. He even pushed for orthomolecular psychiatry, advocating that “orthomolecular therapy, the provision for the individual person of the optimum concentrations of important normal constituents of the brain, may be the preferred treatment for many mentally ill patients.”

He also began making claims that mega-doses of vitamin C could be used to treat or prevent cardiovascular disease and the common cold. Despite the lack of robust evidence that it works, Pauling’s claims remain a standard in the alternative medicine world.

Pauling then moved into the world of vitamin C and cancer, claiming that it could treat or prevent cancer. He published two studies (here and here) that claimed that vitamin C increased survival by as much as four times (compared to untreated patients) in a group of 100 terminal patients.

The studies were roundly criticized by other researchers. An evaluation of Pauling’s research found that the patient groups were different, which lead to a high amount of bias. The vitamin C group was actually less sick than the control group upon entering the study.

Furthermore, researchers at Mayo Clinic published a well-controlled trial in 1979, along with another in 1985, which established that high-dose vitamin C was no better than a placebo for treating cancer. Both studies concluded that there was no benefit to high-dose vitamin C for cancer.

Despite the near-total debunking of Pauling’s claims about vitamin C by legitimate scientific evidence, the legend persists. Part of this persistence is a result of boatloads of laboratory research out there that seem to hit the news every month.

Photo by riccardo ragione on Unsplash

Vitamin C and cancer –  preclinical research

Preclinical research is a body of biomedical research that relies on animal or in vitro models. This type of primary research is generally of low quality in the hierarchy of biomedical research.

Only a small percentage of drugs (and let’s be clear, using megadoses of vitamin C is a drug) that show promise in preclinical research end up having any clinical utility. Less than 13% of drugs that enter clinical trials ever receive FDA approval.

There is a tendency for the press to overstate the results from preclinical research, so we quickly translate a study that shows that vitamin C in a cell culture treats a particular study to the headline “Vitamin C cures cancer again.”

Recently, two studies (here and here) seem to indicate that vitamin C could prevent leukemia. To be fair, the NBC News article was careful in its presentation of the data, but it started the article with “Here’s another reason to make sure you’re eating plenty of vitamin C.”

First of all, these studies looked at a very specific type of leukemia where a mutation in the TET2 gene leads to certain cancers, including acute myeloid leukemia. Vitamin C mimics TET2 restoration, so it helps other cancer drugs attack the disease. And neither article suggested, or even hinted, that megadoses of vitamin C improved the effect of standard chemotherapy. All it said was that maintaining proper vitamin C levels through diet helped chemotherapy.

The researchers did not say that vitamin C treated this cancer directly. They did not say that it could prevent leukemia. They did not say megadoses of vitamin C would do anything.

The National Cancer Institute reviewed the most important preclinical studies, and it is clear that there is no robust evidence that vitamin C, especially in megadoses, has any value.

Many studies showed that vitamin C enhances the effects of some chemotherapy drugs, again in animal and cell culture studies. However, and this is a critical point, there is a large amount of evidence that shows that vitamin C does not have an effect on cancer treatment.

But the only thing that matters regarding vitamin C and cancer are robust and well-designed clinical trials. All of these preclinical studies do not and cannot provide us with any clinical information that would tell an oncologist to suddenly use vitamin C in any therapy.

photo of smiling woman in white and green stripe shirt holding up orange fruits
Photo by Anna Shvets on

Vitamin C and cancer – clinical trials

As I mentioned before, clinical trials tell us whether a medical modality shows a benefit in human beings. And even better, systematic reviews, which compile the data from similar clinical trials, provide us with the best quality evidence about the effectiveness of a potential medical treatment.

Below is a list of these clinical studies and reviews for various cancers that are of the highest quality:

  • In a meta-review of 49 clinical trials involving nutrients and cancer, Schwingshackl et al. stated that “Vitamins D, C, and K; selenium; zinc; magnesium; and eicosapentaenoic acid showed no significant risk reduction for any of the outcomes.” In other words, these nutrients did not reduce the risk of cancer.
  • In a meta-review of 51 studies, Fulan et al. found that there was no change in the risk of breast cancer after the consumption of vitamin C.
  • In a systematic review and meta-analysis of 9 studies involving vitamin C, Papaioannou et al. concluded that vitamin C is “not effective in the chemoprevention of colorectal neoplasia in the general population. This questions their involvement in future randomized controlled trials of chemoprevention in colorectal cancer.” On the other hand, in a meta-review of observational studies, Yu et al. found a “limited association” between dietary vitamin C and colorectal cancer, but again, it is not robust data that would support an evidence-based medicine decision about cancer therapy.
  • In a systematic review of 14 studies of vitamin C and prostate cancer, Stratton and Godwin determined that “there is no convincing evidence that the use of supplemental multivitamins or any specific vitamin affects the occurrence or severity of prostate cancer.”
  • In a Cochrane Review of research on nutrients in lung cancer in healthy individuals, Cortés-Jofré et al. determined that “there is no evidence for recommending supplements of vitamins A, C, E, selenium, either alone or in different combinations, for the prevention of lung cancer and lung cancer mortality in healthy people.” However, another review stated that there may be a limited protective effect of vitamin C against lung cancer in smokers. But the authors clearly stated that the best protective effect was to stop smoking.
  • Wilson et al. concluded that “the use of high-dose IV vitamin C (on any cancer) cannot be recommended outside of a clinical trial.”
  • A systemic review of several clinical trials “does not prove that there is a clinically relevant positive effect of vitamin C supplementation in cancer patients in general on the overall survival, clinical status, quality of life (QOL), and performance status (PS).”
  • A meta-analysis of vitamin C levels and risk of five different cancers in Europeans found that “these findings provide no evidence to support that physiological-level circulating vitamin C has a large effect on the risk of the five most common cancers in European populations.”

I could go on and on. Nearly every systematic review and meta-analysis, both of which are at the very top of the hierarchy of clinical research, show that there is simply no link between vitamin C levels or excess supplementation and the treatment or prevention of cancer. I got bored of reading one after another review that just showed nothing.

Most individual clinical studies that examined the effect of vitamin C on a variety of cancers have been small in size and have had a very limited effect on the prognosis of the disease. The weakness of these types of studies contributes to an overall skepticism about any beneficial effect of vitamin C on cancer.

For example, in a 2014 Phase I clinical trial, Ma et al. evaluated the toxicities of combining IV vitamin C (ascorbate) with typical chemotherapy agents for ovarian cancer. Twenty-seven patients were randomly assigned to receive either chemotherapy alone or chemotherapy and IV vitamin C concurrently. They concluded that the addition of IV vitamin C was associated with reduced chemotherapy-related toxicities.

However, David Gorski took a different view of this study in Science-Based Medicine. He had several criticisms:

  1. Bad statistics might lead to numerous false positives.
  2. The high amount of vitamin C delivered intravenously far exceeds the levels of chemotherapy used.
  3. The study was tiny and not randomized or blinded. According to Gorski, “There would have had to have been an enormous effect to produce a statistically significant effect on survival or progression in such a small study.”
  4. There was no difference between the two groups for the most severe forms of toxicity. In other words, their conclusion that vitamin C was associated with reduced chemotherapy-related toxicities was not supported by the evidence.

Vitamin C and cancer are one of those alternative medicine icons that have never been shown to be supported by real scientific evidence. Yet, how often do you hear that vitamin C is the thing to prevent cancer? I think I see it every day, and I just thought it had disappeared into the detritus of the internet. I am wrong.


Is there much evidence that vitamin C prevents cancer? Not much at all.

Is there much evidence that vitamin C can treat cancer? Again, there is mostly none.

To quote Dr. Gorski from the aforementioned article from 2014 (meaning vitamin C and cancer have been debunked for so many years that it should be right up there with Sasquatch as real science):

This latest highly unimpressive study being touted as evidence that high dose intravenous ascorbate/vitamin C therapy is anything other than a long run for a short slide is merely part of the campaign to insinuate quackademic medicine even more firmly into the mainstream than it has regrettably already succeeded in doing.

That’s about as clear as we can get regarding vitamin C and cancer. There’s not much there.

Oh, and in case you’re wondering, vitamin C is also useless against colds and flu. Again, in case you were wondering.


Michael Simpson

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