Vitamin B12 and Cancer Risk — Why More Is Not Always Better

Vitamin B12 has long been celebrated as a nutritional essential, critical for DNA replication, red blood cell formation, and nervous system health. But a growing body of research is prompting scientists to ask harder questions about whether high-dose supplementation truly benefits health — or could, in some contexts, cause harm.

A 2025 case-control study from Vietnam identified a U-shaped relationship between B12 intake and cancer risk. Both low and high B12 intake were associated with elevated cancer risk, suggesting that optimal intake exists within a defined range. While this type of observational study cannot establish cause and effect, it adds to a pattern of evidence suggesting that extremes in either direction carry risk.

The biological concern with very high B12 centers on cell growth. B12 supports DNA synthesis and cell division broadly — not selectively in healthy cells. In theory, if pre-cancerous cells are already present, an abundance of growth-supporting nutrients could accelerate their development. This remains difficult to definitively prove in human studies, but the hypothesis is scientifically plausible.

Long-term, high-dose B vitamin supplementation has not demonstrated consistent cancer-protective effects in clinical research. Some observational data point to a modest increase in lung cancer risk associated with high-dose B6 and B12 use, especially in male smokers. Meanwhile, elevated blood B12 commonly seen in cancer patients appears to be a consequence of the disease — an epiphenomenon — rather than a driver, according to a 2022 analysis.

The practical message for health-conscious adults is one of calibration rather than fear. People at genuine risk of deficiency — vegans, older adults, those with gut absorption issues — still benefit from supplementation. But routine high-dose B12 supplementation without confirmed deficiency lacks strong evidence of benefit and may carry underappreciated risks worth discussing with a clinician.