Vitamin C, also known as ascorbic acid, has sparked controversy since it first emerged as a potential anti-cancer agent. However, an increasing number of preclinical studies have demonstrated that high-dose vitamin C exhibits selective anti-tumor effects, including “pro-oxidative cytotoxicity”, “anti-cancer epigenetic regulation”, and “immune modulation”. Consequently, vitamin C has reemerged as a promising anti-cancer therapy in the form of high-dose administration. Advancements in pharmacokinetic research have facilitated the development of clinical trials. Early clinical studies across various cancer types have confirmed the safety of high-dose vitamin C administered via intravenous injection. Moreover, its use as an adjuvant therapy in combination with standard treatments, such as chemotherapy and radiotherapy, has shown promising therapeutic potential. However, there remains a lack of consensus regarding optimal dosage, administration methods, tumor specificity, and patient selection. These factors have contributed to the inconsistent outcomes observed in phase Ⅱ clinical trials and have hindered the widespread conduct of phase Ⅲ trials. Without robust clinical evidence, high-dose vitamin C, despite being a non-toxic and promising anti-cancer agent, risks being “shelved” once again. In this review, we provide a comprehensive overview of the anti-tumor mechanisms of high-dose vitamin C and a detailed analysis of preclinical and clinical studies investigating its role as an anti-cancer agent. Additionally, we explore emerging trends in high-dose vitamin C therapy for cancer treatment and offer recommendations for future research in this field.