When people don’t have enough vitamin B12, their blood tends to have less hemoglobin — which means their blood can’t carry oxygen as well. The less B12 they have, the worse their anemia seems to be.

If someone has low vitamin B12 and anemia, their red blood cells are unusually large and their blood doesn't carry enough oxygen. Giving them a weekly B12 shot for six weeks can fix this, making their blood cells normal size again and boosting their oxygen levels.

People in India who go gray too early are more likely to have low levels of vitamin B12, and some of them also have a sign in their blood that might mean their body is having trouble absorbing this vitamin.

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.

When adults don’t have enough vitamin B12, their body makes fewer new red blood cells. But after getting weekly B12 shots for six weeks, their body starts making many more new red blood cells again, which shows their bone marrow is working better.

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.