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.

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.

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.

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.

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.