Vitamin B12 is a water-soluble vitamin. One of the man-made versions of injectable B12 is called Hydroxocobalamin and this is what is often injected to treat low levels of B12. B12 helps your body to use fat and carbohydrate for energy and it also helps to make protein. It is also very important for blood, nerves and cells. It is mainly taken into the body by a diet that is rich in meat, fish, eggs, dairy products, yeast extract (such as marmite) and foods that have been fortified with B12 such as some plant-based milk. Hydroxocobalamin is the most commonly used form of B12 and is the preferred type by the World Health Organisation (WHO). This is because it stays in the body for longer than some other forms of B12, which means fewer injections over time. Injected B12 begins to work straight away but it can be days or weeks before you begin to feel better.