For people with type 2 diabetes, having too little or too much vitamin B12 in the blood might increase the chance of dying from heart disease — the safest range seems to be in the middle.

In people with type 2 diabetes, having very high levels of vitamin B12 in the blood seems to be linked to a much higher chance of dying from heart disease—higher than at any other B12 level—so maybe high B12 isn’t helping, but instead signals something else 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 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 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 very high levels of vitamin B12 in their blood (above 1000 pg/L) seem to have a higher chance of getting certain cancers, like lung, pancreatic, liver, or blood cancers, based on studies that found their risk was nearly twice to almost six times higher.