Vitamin C has been a subject of interest in cancer therapy due to its potential effects on cancer cells and the immune system. This review explores the role of vitamin C in cancer therapy, focusing on clinical trials and its efficacy. The review includes a discussion of the clinical trials on vitamin C in cancer therapy. Other results showed no significant difference in cancer prevalence between the intervention and control groups, suggesting that vitamin C intake did not affect cancer incidence. The study found that high-dose intravenous vitamin C in combination with chemotherapy and radiation therapy was safe and well-tolerated, with no significant adverse effects. However, the study did not show a significant effect on overall survival or progression-free survival. Despite the lack of significant results in clinical trials, there is evidence to support the use of vitamin C as an adjuvant in cancer therapy. Vitamin C has been shown to have anti-tumor effects, including the creation of reactive oxygen species (ROS) that directly have cytotoxic activity on cancer cells. Additionally, vitamin C can create important epigenetic changes due to its effect on 2-oxoglutarate-like histone and DNA de-methylated. In preclinical studies, vitamin C has been shown to have a synergetic effect with some types of chemotherapy and radiation therapy. Additionally, vitamin C has been shown to stimulate the production and activation of immune cells, such as T-lymphocytes and natural killer cells, which have a function in fighting against pathogens and cancer cells. In conclusion, while clinical trials have not shown significant effects of vitamin C on cancer prevention or therapy, there is evidence to support the use of vitamin C as an adjuvant in cancer therapy. Further research is needed to determine the optimal dose and route of administration of vitamin C in cancer therapy.