High vitamin B12 on a blood test linked to cancer? Four million records suggest it is mostly about timing

Vitamin B12 is one of the most often ordered blood tests, and both low and unusually high results have been linked with cancer and death. A study published in The Journal of Nutrition on 29 September 2026, led from National Cheng Kung University in Taiwan, used health records of 3,950,497 adults from the TriNetX network (169 healthcare organisations in 19 countries, 2006 to 2020) to look at the shape of that link over 10 years.

Compared with people whose B12 was 400 to 600 pg/mL, cancer was modestly more common at higher levels (adjusted hazard ratios of 1.05 to 1.08), with no link in the deficiency range. Deaths from any cause followed a J-shape, higher at both extremes. The key test was timing: when the first two years of follow-up were left out, the cancer link for levels above 1,000 pg/mL shrank from 1.07 to 1.02. That fits the idea that some high B12 readings come from an illness that is already present, rather than B12 causing cancer. The higher death rate with low B12 (below 200 pg/mL) faded far less, from 1.26 at one year to 1.18 at ten years.

The authors stress that these observational results do not justify stopping B12 treatment that a doctor has prescribed. An unexplained high B12 without supplements is worth discussing with your doctor; it is not in itself a cancer diagnosis.