Vitamins

Key examples

• Folic acid
• Thiamine
• Hydroxocobalamin
• Phytomenadione

 

Common indications

• Thiamine (vitamin B1) is used in the treatment and prevention of Wernicke’s encephalopathy and Korsakoff’s psychosis, which are manifestations of severe thiamine deficiency.
• Folic acid (the synthetic form of folate or vitamin B9) is used in megaloblastic anaemia due to folate deficiency, and in the first trimester of pregnancy to reduce the risk of neural tube defects.
• Hydroxocobalamin (a synthetic form of cobalamin or vitamin B12) is used in the treatment of megaloblastic anaemia and subacute combined degeneration of the cord due to vitamin B12 deficiency.
• Phytomenadione (the plant form of vitamin K) is recommended for all newborn babies to prevent vitamin K deficiency bleeding, and is used to reverse the anticoagulant effect of warfarin (prothrombin complex concentrate should also be given in cases of major bleeding).

 

Mechanisms of action

• Vitamins are organic substances required in small amounts for normal metabolic processes. 
• Vitamin deficiencies and their associated clinical manifestations may be treated with a pharmaceutical form of the relevant vitamin. 
• In pregnancy and the preconception period, giving folic acid reduces the risk of congenital neural tube defects. As it is required for normal cell division, it may work by facilitating cell proliferation involved in neural tube closure, but this is not completely understood. 
• Phytomenadione reverses warfarin by providing a fresh supply of vitamin K for the synthesis of vitamin K-dependent clotting factors by the liver.

 

Important adverse effects

• When given IV, phytomenadione and high-dose thiamine may rarely cause anaphylaxis. 
• Most other vitamin preparations are relatively non-toxic.

 

Warnings

• In patients with co-existing B12 and folate deficiency, you should replace both vitamins simultaneously. 
• This is because replacing folic acid alone may be associated with (and perhaps hasten) progression of the neurological manifestations of B12 deficiency. The major concern is the risk of provoking subacute combined degeneration of the cord. 
• Phytomenadione is less effective in reversing warfarin in patients with severe liver disease, as clotting factors are synthesised in the liver.

 

Important interactions

• As noted above, vitamin K and warfarin have an antagonistic interaction which, initially, is desirable. However, when attempting to restart warfarin after vitamin K has been given, it may result in erratic dosing requirements.