Too little Vitamin B12
Vitamin B12 (Cyanocobalamin) is under normal healthy conditions produced in sufficient quantities by the large intestine of the horse.
To aid in its absorption Vitamin B12 needs a carrier protein produced by the stomach wall. Diseases of the gastric mucosa (gastroenteritis, parasitics infestations) may give secondary anaemias due to vitamin B12 deficiency.
Solutions: Provide feeds supplemeted with vitamin B12. (Also ensure that a correct worming programme is strictly observed).
Too much Vitamin B12
Over feeding of vitamin B12 (Cyanocobolamin) has no known side effects.
Solutions: Consider reducing intake of high B12 feeds and supplements.