Abstract Background and Aim: DNA synthesis and cellular metabolism depend on vitamin B12. Emerging research indicates that excessive supplementing may raise the risk of cancer, even as moderate doses may offer protection against some malignancies. The preventive and promoting effects of vitamin B12 on cancer are assessed in this systematic study. The aim of the study is to synthesise existing evidence to clarify the complex association between Vitamin B12 intake, serum levels, and cancer risk. Methods: Clinical trials and observational studies published in English were found by a thorough search of major scientific databases. Only abstracts, in vitro experiments, and animal studies were not included. Four of the 300 articles that were screened satisfied the requirements for eligibility. Serum B12 levels, dietary intake, supplementation, and cancer-related outcomes were also examined. The OHAT technique was used to evaluate the risk of bias. Results: A higher incidence of colorectal and total cancer was linked to high-dose supplementation (≥500 μg/day). Serum B12 levels in cancer patients were significantly higher (>800 pg/mL) and were correlated with tumor stage. On the other hand, a decreased risk of colorectal cancer was associated with a sufficient dietary B12 consumption from natural food sources. A lower risk of oesophageal precancerous lesions was likewise linked to higher levels of transcobalamin II and serum B12. Conclusion: There are two connections between vitamin B12 and cancer. While avoidable long-term high-dose supplementation may raise the risk of cancer, maintaining appropriate physiological levels is advantageous. Until a clinical deficiency is established, dietary sources should be given priority.