Vitamin and mineral deficiencies are usually caused by low dietary intakes or by certain medical conditions, such as those affecting absorption from the gut. Once a deficiency is diagnosed, it is often a straight-forward case of replacing the missing vitamin or mineral with a supplement. Iron tablets, calcium tablets, vitamin D tablets and vitamin B tablets are all commonly used supplements. Hydroxocobalamin is a vitamin B12 injection given to people to prevent or treat certain types of anaemia. Folic acid is often used to prevent neural tube defects occurring during pregnancy.