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.

People with higher levels of vitamin B12 in their blood tend to get breast cancer less often than those with lower levels.

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.

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.

If your vitamin B12 levels are way higher than normal for no clear reason, doctors should check for possible cancer, because it might be a sign something serious is going on inside your body.

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.