Oral or intravenous vitamin C (ascorbate or ascorbic acid) is a popular adjunctive treatment among patients with cancer, some of whom believe it may have anticancer properties, improve the efficacy of conventional treatment, and/or reduce toxicities. These beliefs are, however, based primarily on data from case reports and early uncontrolled studies.1

A systematic review of research into the relationship between vitamin C and cancer consisted of 5 randomized controlled trials, 12 phase 1/2 trials, 6 observational studies, and 11 case reports. In this review, vitamin C did not result in significant changes in progression-free survival, overall survival, or toxicity compared with control arms.

Some efficacy was noted in observational studies conducted in the 1970s and 80s, and some studies demonstrated improvements in quality of life scores. Adverse event (AE) reports ranged from no significant toxicity to grade 3 and grade 4 events. The randomized controlled trials in this systematic review are outlined below.

Efficacy as Anticancer Therapy

In a double-blind study published in 1979, 150 patients with advanced cancer were randomly assigned to receive 10 g per day (as 4 doses) of vitamin C or placebo until death or inability to take oral tablets. Cancer types included colorectal, pancreas, lung, and stomach, among others. There was no significant difference in symptoms or survival between the vitamin C and placebo arms. The most common AEs were nausea and vomiting, which was similar between arms.2

Another double-blind trial randomly assigned 100 patients with advanced colorectal cancer and no prior chemotherapy to receive 10 g per day (as 4 doses) of vitamin C or placebo. There was no significant difference in survival and no patient demonstrated tumor regression. Compliance was assessed by diary and found to be excellent. Treatment was well-tolerated, though there was a non-significant increase in pyrosis with vitamin C.3

A trial not included in the systematic review evaluated the effect of vitamin C on prostate-specific antigen (PSA) levels among men with hormonally untreated prostate cancer. The trial randomly assigned 80 patients with confirmed prostate cancer and PSA greater than 4 ng/mL to receive placebo or supplementation with 750 mg vitamin C, 200 μg selenium, 350 mg vitamin E, and 200 mg of coenzyme Q10 once per day for 21 weeks. There was no significant change from baseline in PSA serum levels between treatment groups after the intervention.4