“Vitamin C has a negative effect on cancer outcomes.”

When IV vitamin C is combined with certain anticancer drugs, the anticancer drugs may not work as well. So far, these effects have been seen only in some laboratory and animal studies. IV vitamin C has caused very few side effects in clinical trials. However, IV vitamin C may be harmful in people with certain risk factors, such as kidney disease, G6PD deficiency, or hemochromatosis.

Vitamin C intake was negatively correlated with risks of oral, pharyngeal, and esophageal cancers (0.81, 95% CI 0.72–0.93), gastric cancer (0.81, 95% CI 0.68–0.95), and colorectal cancer (0.89, 95% CI 0.82–0.98). These estimates suggest that vitamin C intake could significantly reduce gastrointestinal cancer incidence.

In vitro, pharmacological doses of Vit-C from 0.3–20 mmol/L preferentially target and kill cancer cells, compared to the usual physiological levels of Vit-C, which are 0.1 mmol/L. This tumor-killing phenomenon is due to the pro-oxidant characteristics of Vit-C, which, at high concentrations, facilitates the formation of hydrogen peroxide (H2O2), which may be an agent responsible for the anti-tumor impact of Vit-C and its use as a pro-drug in cancer therapies.

One hundred and fifty patients with advanced cancer participated in a controlled double-blind study to evaluate the effects of high-dose oral vitamin C (10 g per day) on symptoms and survival. The two groups (vitamin C vs. placebo) showed no appreciable difference in changes in symptoms, performance status, appetite or weight, and the median survival for all patients was about seven weeks, with survival curves essentially overlapping.

There's still no evidence that vitamin C alone can cure cancer, but researchers are studying whether it might boost the effectiveness of other cancer treatments. Initial studies in humans had promising results, but these studies were later found to be flawed. Subsequent well-designed, randomized, controlled trials of vitamin C in pill form found no such benefits for people with cancer.

This umbrella review found that Vitamin C (VC) consumption was associated with lower incidence of bladder cancer, breast cancer, cervical tumors, endometrial cancer, esophageal cancer, gastric cancer, glioma, lung cancer, pancreatic cancer, prostate cancer, renal cell cancer, and total cancer occurrence. VC intake was also related to decreased risk of breast cancer prognosis (recurrence, cancer-specific mortality, and all-cause mortality).

Combinational chemotherapy with ascorbic acid and paclitaxel not only does not block the anticancer effects of paclitaxel but also alleviates the cytotoxicity of paclitaxel in vivo and in vitro. In H1299 cells, the anticancer effects of the combinational treatment were up to 1.7-fold higher than those of single-agent paclitaxel treatment.

High pharmacological doses of vitamin C may induce prooxidant effects, detrimental for cancer cells. Intravenous administration of ascorbic acid at high concentrations was toxic for many types of cancer cells in xenografts in mice with no effect on normal cells. The authors suggested that ascorbic acid could support the formation of hydrogen peroxide in cancer cells leading to oxidative stress and cell death.

Antioxidant supplements may interfere with chemotherapy's ability to kill cancer cells. Some studies suggest a link between antioxidant use during chemotherapy and reduced treatment efficacy. High doses of vitamin C might make some chemotherapy less effective, for example, research found that vitamin C can lower the effectiveness of Tamoxifen, a breast cancer drug.

The results of the phase 2 trial—published in the November 2024 issue of Redox Biology—found that the patients' survival rates doubled from eight months to 16 months, and progression-free survival stretched from three months to six. 'When we started the trial, we thought it would be successful to get to 12 months of survival. Instead, we doubled it,' says Joseph Cullen (86MD, 91R), professor of surgery and radiation oncology, and the study's senior author.

While nutritional VC intake demonstrates chemopreventive effects, high-dose intravenous VC acts as a prooxidant to selectively kill tumor cells via ROS-mediated DNA damage. Importantly, VC synergizes with conventional therapies by radio-sensitizing hypoxic tumors and reversing platinum resistance, and early-phase clinical trials corroborate VC's safety profile and potential to ameliorate chemotherapy-induced fatigue and nephrotoxicity.

In pre-clinical studies, vitamin C appears to substantially reduce the effectiveness of anticancer drugs, say researchers at Memorial Sloan-Kettering Cancer Center. The new findings, published in the October 1 issue of Cancer Research, came from studying laboratory cancer cells and mice, but the study's authors say the same mechanism may affect patient outcomes, although they add this premise needs to be tested.

There are some concerns that large doses of vitamin C might reduce the effectiveness of certain cancer treatments, such as chemotherapy and radiation. A recent review of 38 studies on vitamin C and cancer in general concluded that vitamin C cannot treat or prevent cancer.

Other studies support the idea that vitamin C is inversely associated with lowered cancer risk. Women in one study who consumed the lowest amount of vitamins in their diets displayed more cases of esophageal cancer than men, with 68% of women having vitamin C intake below the RDA compared to only 55% of men.

VC consumption was associated with lower incidence of bladder cancer, breast cancer, cervical tumors, endometrial cancer, esophageal cancer, gastric cancer, glioma, lung cancer, pancreatic cancer, prostate cancer, renal cell cancer, and total cancer occurrence. VC intake was also related to decreased risk of breast cancer prognosis (recurrence, cancer-specific mortality, and all-cause mortality).

Cameron and Pauling reported in 1976 and 1978 that high-dose vitamin C (typically 10 g/day, by intravenous infusion for about 10 days and orally thereafter) increased the average survival of advanced cancer patients and for a small group of responders, survival was increased to up to 20 times longer than that of controls.

Many studies have indicated that taking antioxidant supplements during chemotherapy and/or radiation treatment was associated with a lower breast cancer survival rate and may hasten the spread of lung cancer, as well as some other cancers. One study observed that taking vitamin C with Tamoxifen decreased its effectiveness.

Large prospective cohort studies and meta-analyses consistently show that higher dietary vitamin C intake is associated with reduced risk of multiple cancer types, including lung, gastric, and colorectal cancers. This inverse association has been observed across diverse populations and study designs.