A daily vitamin C pill did not slow a blood condition that can turn into leukemia, but people who took it were more likely to be alive three years later

Some older adults are told their blood counts show early warning signs of leukemia, and that the only option is to wait and watch. These patients also tend to have low vitamin C levels. Researchers in Denmark and the United States tested whether a daily vitamin C pill could help, in the first double-blind, placebo-controlled trial of its kind. They gave 109 patients either 1,000 mg of vitamin C or a placebo every day for a year and followed them for about three years.

Vitamin C did not do the main thing the researchers hoped it would. It did not significantly slow the growth of the abnormal blood cells that can eventually turn into leukemia.

But an exploratory analysis found something else. Patients who took vitamin C were significantly more likely to be alive about three years later than those who took the placebo. They also had fewer adverse events during the study and follow-up, and their levels of certain immune signaling molecules shifted in ways linked to better outcomes.

“It is too early to make definitive recommendations based on our results, but we are hopeful that a more expansive study will provide further insights,” said Kirsten Grønbæk of the University of Copenhagen, who led the trial.

The patients had either clonal cytopenia of undetermined significance or lower-risk myelodysplastic syndrome. Both are blood disorders, more common with age, that can progress to acute myeloid leukemia, an aggressive cancer. There is no approved treatment to stop that progression in the first group, and many patients in the second are too old or unwell for a stem cell transplant.

More than half of the patients had insufficient vitamin C at the start. In the vitamin C group, levels returned to normal. The dose, 1,000 mg a day, is in the range of many over-the-counter supplements.

Vitamin C and cancer have a long and controversial history. In 1976, Linus Pauling and Ewan Cameron reported in a study in PNAS that high-dose vitamin C appeared to extend survival in terminal cancer patients. When doctors at the Mayo Clinic tested oral vitamin C in a randomized trial published in 1979, they found no benefit, and the idea lost credibility for decades.

The new rationale is different. Vitamin C helps an enzyme called TET2 keep genes switched on or off correctly, and TET2 mutations are common in these blood disorders and in leukemia. Lab studies found vitamin C worked best before cancer took hold, which is why the team tested it at this early stage.

“The standard approach for patients with pre-cancer blood disorders is to monitor their blood cell counts over time. This is, understandably, very frustrating,” said Casey O’Connell of the University of Southern California, who led the trial site there.

For anyone with these conditions, the findings are a reason to ask a hematologist about vitamin C levels and about upcoming trials, not to start a high-dose supplement on your own. Supplements often look promising until they are tested properly, as a recent fish oil trial showed.

There are limits to what this shows. With 109 patients, the trial was small, and the survival result came from an exploratory analysis rather than the trial’s main question, so it may not hold up. The researchers say a larger phase 3 trial is needed. The study was funded by the Van Andel Institute, Stand Up To Cancer, the Danish Cancer Society and Greater Copenhagen Health Science Partners.

Source

Mikkelsen SU, Al-Mousawi A, Puglisi AB, et al. “Oral vitamin C supplementation in patients with clonal cytopenia of undetermined significance or lower-risk myeloid malignancies: Results from EVITA, a phase 2 randomized, placebo-controlled trial.” Cancer, 2026.
DOI: 10.1002/cncr.70549

Also cited

Cameron E, Pauling L. “Supplemental ascorbate in the supportive treatment of cancer: prolongation of survival times in terminal human cancer.” Proceedings of the National Academy of Sciences, 1976.
DOI: 10.1073/pnas.73.10.3685

Creagan ET, Moertel CG, O’Fallon JR, et al. “Failure of high-dose vitamin C (ascorbic acid) therapy to benefit patients with advanced cancer: a controlled trial.” New England Journal of Medicine, 1979.
DOI: 10.1056/NEJM197909273011303