Scientists have issued a warning about vitamin B12 supplementation, advising against 'megadosing' after new research suggested a potential link between high levels and cancer. B12 is essential for red blood cell production, nervous system function, and DNA repair, but both too little and too much may pose risks.
The Importance of Balance
B12 is naturally found in animal products like meat, fish, eggs, and dairy, as well as fortified cereals. Most people get enough from a varied diet, but vegans, older adults, and those with gut conditions may need supplements. However, a 2025 case-control study from Vietnam found a U-shaped relationship between B12 intake and cancer risk, meaning both low and high intakes were associated with increased risk.
Cancer Link Explained
Writing on The Conversation, experts Ahmed Elbediwy and Nadine Wehida from Kingston University explained that B12 is critical for cell division and DNA copying. Deficiency can lead to mutations that may increase colon cancer risk over time. Conversely, very high levels of B12 might support the growth of pre-cancerous cells, though this remains unproven in humans.
Long-term high-dose B vitamin supplements have not shown clear protective effects against cancer. Some observational research even suggested a slight increase in lung cancer risk among men and smokers taking high-dose B6 and B12, though causation is not established.
B12 as a Marker, Not a Cause
Doctors have noted that elevated B12 levels often appear alongside cancer, but research in 2022 indicated it is a marker rather than a trigger. A 2026 study found that colon cancer patients with very high B12 levels survived a median of five years, compared to nearly eleven years for those with normal levels. Similar patterns were seen in oral cancer and immunotherapy patients.
Symptoms of Deficiency
Vitamin B12 deficiency can cause symptoms like rapid breathing, headaches, indigestion, loss of appetite, palpitations, vision problems, weakness, diarrhea, a sore tongue, memory issues, and neurological problems including numbness, muscle weakness, depression, confusion, and balance issues.
Key Takeaway
The scientists stress that 'more is not better.' For most people, getting B12 from food is sufficient. Prolonged high-dose supplementation without medical advice is discouraged. Unexplained persistent high B12 levels should be investigated, as they may indicate liver disease, blood disorders, or undetected cancer.
They conclude: 'Get enough through food or supplementation if you need it, especially if you are vegan, older or have a condition that affects absorption. But leave the megadoses on the shelf unless a doctor advises them. With B12, as with many nutrients, the goal is not as much as possible. It is the right amount.'
