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

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.

When someone has cancer, their vitamin B12 levels sometimes go up—but it’s not clear if that’s because the cancer is causing it, or if the high B12 is just a side effect and not actually helping diagnose anything.

Right now, scientists haven’t found a clear link between how much vitamin B12 is in your blood and whether you’re more or less likely to die from cancer.

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.

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.