Haematology/B12 & folate (megaloblastic anaemia)

Vitamin B12 & folate

also: hydroxocobalamin · folic acid · pernicious anaemia · megaloblastic

Overview

Replacement for megaloblastic anaemia. The order rule — B12 BEFORE folate — prevents precipitating subacute combined degeneration of the cord.

Mechanism

Both are cofactors for DNA synthesis; deficiency → megaloblastic anaemia. B12 is also needed for myelin — so giving folate alone in combined deficiency can worsen the neurological lesion (subacute combined degeneration).

Indications

B12 deficiency / pernicious anaemia (IM hydroxocobalamin)
Folate deficiency (oral folic acid)
Folic acid: pregnancy NTD prevention (cross-ref O&G)

The agents

Hydroxocobalamin (B12)IM

loading then 3-monthly

Pernicious anaemia (lifelong); also high-dose IV in cyanide poisoning.

Folic acidoral

Folate deficiency; NTD prevention (preconception).

Adverse effects

Giving folate before correcting B12 → subacute combined degenerationserious

CHECK/replace B12 first if both deficient.

Generally well toleratedcommon

Cautions & contraindications

Folate alone when B12 status unknown/lowall

Can precipitate/worsen SACD.

Interactions

Methotrexate, trimethoprim, phenytoin lower folate

Monitoring & kinetics

FBC/reticulocytes; B12/folate levels; neurological status

B12 IM; folate oral. Treat B12 first in combined deficiency.

Source: BNF — Anaemias - megaloblastic