Vitamin C: Oral Use

Vitamin C, available in several foods and dietary supplements, shows some anticancer effects, including better survival among people with breast cancer.

Safety and precautions

Overall, oral vitamin C has a good safety profile.¹Mazloom Z, Ekramzadeh M, Hejazi N. Efficacy of supplementary vitamins C and E on anxiety, depression and stress in type 2 diabetic patients: a randomized, single-blind, placebo-controlled trial. Pakistan Journal of Biological Sciences. 2013 Nov 15;16(22):1597-600; Yeom CH, Jung GC, Song KJ. Changes of terminal cancer patients’ health-related quality of life after high dose vitamin C administration. Journal of Korean Medical Science. 2007;22(1):7-11. However, several precautions are noted here.

Increased cancer risk

Evidence shows higher risk of liver cancer or melanoma among people with higher intake of vitamin C, sometimes with other antioxidants, and possibly slightly higher risk of recurrence or diagnosis of breast cancer, rectal cancer, and gastrointestinal cancer. 

• Weak trendsan apparent change due to a therapy, close to but not achieving full statistical significance (this is the CancerChoices definition; other researchers and studies may define this differently toward increased risk of recurrence and death among people with breast cancer taking any antioxidants (vitamins C, A, and E; carotenoids; or coenzyme Q10) during chemotherapy compared to no antioxidants in a large observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study²Ambrosone CB, Zirpoli GR et al. Dietary supplement use during chemotherapy and survival outcomes of patients with breast cancer enrolled in a cooperative group clinical trial (SWOG S0221). Journal of Clinical Oncology. 2020;38(8):804-814.
• Higher risk of breast cancer among people with genetically predicted higher levels of circulating vitamin C levels compared to lower levels in one large cohort (UK Biobank) but no evidence of an effectoverall, one or more studies did not demonstrate that a treatment or intervention led to an expected outcome; this does not always mean that there is no effect in clinical practice, but that the studies may have been underpowered (too few participants) or poorly designed. Larger, well-designed studies provide more confidence in making assessments. on risk in other large cohorts (ILCCO and PRACTICAL consortium)³Fu Y, Xu F et al. Circulating vitamin C concentration and risk of cancers: a Mendelian randomization study. BMC Medicine. 2021;19(1):171.
• Weak trends toward higher risk of rectal cancer among people with higher levels of vitamin C intake from supplements compared to lower levels in a very large observational study⁴Fu Y, Xu F et al. Circulating vitamin C concentration and risk of cancers: a Mendelian randomization study. BMC Medicine. 2021;19(1):171.
• Slightly higher gastrointestinal cancer mortality among people taking antioxidant supplements (beta-carotene, vitamins A, C, E, and/or selenium, and especially beta-carotene) compared to placebosa pill, medicine, or procedure—thought to be both harmless and ineffective—prescribed for the psychological benefit to the patient or as a sham treatment in a study to allow a comparison to a therapy of interest in a very large meta-analysisa statistical analysis that combines the results of two or more research studies; the results of smaller research studies addressing the same or similar questions can be analyzed as though they are one bigger, more powerful study of 7 high-quality RCTsrandomized controlled trial, a study design in which people are randomly assigned to either an experimental group or a control group to compare the outcomes from different treatments; an RCT is considered a strong design for determining a therapy’s effects⁵Bjelakovic G, Nikolova D, Simonetti RG, Gluud C. Antioxidant supplements for prevention of gastrointestinal cancers: a systematic review and meta-analysis. Lancet. 2004;364(9441):1219-1228.
• Almost double the risk of liver cancer among people taking vitamin C supplements for cancer prevention, with increasing risk at higher doses, in a very large observational study⁶Zhang W, Shu XO et al. Vitamin intake and liver cancer risk: a report from two cohort studies in China. Journal of the National Cancer Institute. 2012;104(15):1173-1181.
• 43% higher risk of melanoma among people eating foods including orange juice containing vitamin C at levels at or above 175 mg per day compared to below 90 mg per day in a very large observational study⁷Feskanich D, Willett WC, Hunter DJ, Colditz GA. Dietary intakes of vitamins A, C, and E and risk of melanoma in two cohorts of women. British Journal of Cancer. 2003;88(9):1381-1387.

Interactions with cancer treatments

Chemotherapy and radiotherapy

The potential for vitamin C to interfere with some chemotherapy drugs and radiation therapy is a concern. Some researchers even conclude that the use of supplemental antioxidants during chemotherapy and radiation therapy should be discouraged because of the possibility of tumor protection and reduced survival.⁸Lawenda BD, Kelly KM et al. Should supplemental antioxidant administration be avoided during chemotherapy and radiation therapy? Journal of the National Cancer Institute. 2008;100(11):773-783. However, the first author of that study notes in a separate commentary that a review of 19 clinical trials came to a different conclusion about antioxidants and chemotherapy:⁹Lawenda B. Hey Doc, Can I Take Antioxidants During Chemo and Radiation? Integrative Oncology Essentials. March 14, 2013. Viewed June 13, 2023. 

Ladas and Kelly, other integrative medicine experts, discuss the antioxidant debate. They note that while some evidence suggests antioxidants may be beneficial during treatment, other evidence suggests they might reduce treatment efficacy. They point out that not all chemotherapy agents rely on oxidative stress for killing cancer cells, so the risk of antioxidant supplements interacting with chemotherapy is dependent on the type of chemotherapy. Thus uniform recommendations are difficult to make.¹¹Abrams DI, Weil AT. Integrative Oncology, Second Edition. 2014. New York, New York: Oxford University Press.

If you want to explore taking antioxidants such as vitamin C during chemotherapy and radiotherapy, and your medical oncologist isn’t familiar with the evidence, risks, and benefits, you may wish to consult an integrative oncologist who is knowledgeable and experienced about dietary supplementation in cancer. See Finding Integrative Oncologists and Other Practitioners ›

Biopsy and surgical procedures

Since high-dose oral vitamin C thins the blood, experts recommend discontinuing use 5 to 7 days before a biopsy or surgical procedure. 

Side effects or adverse events

The recommended daily amount of vitamin C is 90 milligrams for adult men and 75 milligrams for adult women. Higher doses of oral vitamin C can cause some side effects:¹²Mayo Clinic Staff. Vitamin C. November 17, 2020. Viewed April 7, 2023.

A natural antioxidant at normal blood concentrations, vitamin C at high concentrations in lab studies can become pro-oxidant—interacting with metals, and especially iron and copper, to form harmful oxidant radicals, leading to oxidative stress. Higher levels can also impair the absorption of iron, copper, or vitamin B₁₂ in lab studies.¹³Kaźmierczak-Barańska J, Boguszewska K, Adamus-Grabicka A, Karwowski BT. Two faces of vitamin C—antioxidative and pro-oxidative agent. Nutrients. 2020 May 21;12(5):1501.

Toxic reactions to 10 g oral vitamin C per day included nausea, vomiting, heartburn, diarrhea, and leg swelling among people with advanced cancer.¹⁴Creagan ET, Moertel CG 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; 301(13):687-90.

Short-term high-dose use of oral vitamin C¹⁵Auer BL, Auer D, Rodgers AL. Relative hyperoxaluria, crystalluria and haematuria after megadose ingestion of vitamin C. European Journal of Clinical Investigation. 1998;28(9):695-700. can lead to plasma calcium oxalate supersaturation.

Higher doses of oral vitamin C may raise the risk of kidney stones, although the evidence is mixed.

• Higher risk of kidney stones among Swedish men taking vitamin C supplements, with increasing risk at higher doses, in a very large observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study¹⁶Thomas LD, Elinder CG, Tiselius HG, Wolk A, Akesson A. Ascorbic acid supplements and kidney stone incidence among men: a prospective study. JAMA Internal Medicine. 2013;173(5):386-388.
• 41% higher risk of kidney stones among men consuming 1000 mg or more of vitamin C per day compared with less than 90 mg per day in a very large observational study¹⁷Taylor EN, Stampfer MJ, Curhan GC. Dietary factors and the risk of incident kidney stones in men: new insights after 14 years of follow-up. Journal of the American Society of Nephrology. 2004;15(12):3225-3232.
• No evidence of an effectoverall, one or more studies did not demonstrate that a treatment or intervention led to an expected outcome; this does not always mean that there is no effect in clinical practice, but that the studies may have been underpowered (too few participants) or poorly designed. Larger, well-designed studies provide more confidence in making assessments. on risk of kidney stones among women with the highest intake of vitamin C (1500 mg per day or more) compared to the lowest (less than 250 mg per day) during 6 years of follow up in a very large observational study¹⁸Curhan GC, Willett WC, Speizer FE, Stampfer MJ. Intake of vitamins B6 and C and the risk of kidney stones in women. Journal of the American Society of Nephrology. 1999;10(4):840-5. 
• No evidence of an effect on risk of kidney stones among men with the highest intake of vitamin C (1500 mg per day or more) compared to the lowest (less than 250 mg per day) during 6 years of follow up in a very large observational study¹⁹Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of the intake of vitamins C and B6, and the risk of kidney stones in men. Journal of Urology. 1996;155(6):1847-51.

Interactions with other therapies

At higher doses, vitamin C can interact with other medications.

• A life-threatening iInteraction between high doses of oral vitamin C with amygdalin²⁰Bromley J, Hughes BG, Leong DC, Buckley NA. Life-threatening interaction between complementary medicines: cyanide toxicity following ingestion of amygdalin and vitamin C. Annals of Pharmacotherapy. 2005;39(9):1566-1569.
• A possible and serious interaction between oral vitamin C supplements and warfarin²¹Sattar A, Willman JE, Kolluri R. Possible warfarin resistance due to interaction with ascorbic acid: case report and literature review. American Journal of Health System Pharmacy. 2013;70(9):782-6.
• Interactions with aluminum (common in many antacids), fluphenazine (Prolixin), protease inhibitors for HIV/AIDS, statins for lowering cholesterol, niacin, and other therapies²²Vitamin C. Wellkasa. Viewed April 7, 2023.