People with very high levels of vitamin B12 in their blood—above 600 pmol/L—might be 50% more likely to die from any cause compared to those with lower levels, which could mean there’s a dangerous upper limit to how much B12 is safe.

People with vitamin B12 levels between 400 and 600 units in their blood are 34% more likely to die from any cause than people with lower levels—even though those levels are still considered normal by doctors.

People with higher levels of vitamin B12 in their blood tend to have a slightly higher chance of dying from any cause — especially older adults — so high B12 might just be a sign that something else is wrong in the body.

If your blood has too little or too much vitamin B12, you might be at higher risk of dying from any cause or from heart disease — the safest range is in the middle, between 190 and 948 pg/mL.

People with higher levels of vitamin B12 in their blood seem to have a higher chance of dying from heart problems, but this link isn’t as clear as the link between high vitamin B12 and dying from any cause.

Having higher levels of vitamin B12 in your blood doesn't clearly make you live longer or shorter — studies just can't agree on whether it matters at all.