Vitamin B12 deficiency can occur for several reasons. The most common cause is not eating enough B12-rich foods like meat, fish, dairy, and eggs. Vegans are especially at risk unless they take supplements since B12 is mostly found in animal-based foods. Certain medications, such as anticonvulsants and heartburn drugs, can also interfere with vitamin B12 absorption. Finally, health conditions like autoimmune diseases, digestive issues and stomach problems can reduce B12 absorption.

Some studies suggest that people with excess weight or obesity often have lower vitamin B12 levels, though it's unclear whether low B12 causes weight gain or is simply a result of being overweight. A B12 deficiency can reduce appetite in some people, potentially leading to weight loss. While vitamin B12 supports energy metabolism, its main role is in immune function and red blood cell production so, due to this, does not directly affect weight loss.

Many people feel improvements after taking vitamin B12 supplements within a few days. More noticeable changes, like reduced fatigue, typically take a few weeks. Once B12 levels normalise, you might choose to stop supplementation. However, if your diet lacks sufficient B12 or absorption is an issue, long-term supplementation may be necessary.

Vitamin B12 is generally safe, even at high doses, as your body only absorbs what it needs and excretes the excess.