Remember that it is mandatory to check potassium before treating B12 deficiency.

Explanation

Was I surprised when I learnt this!!

In a patient with B12 deficiency, multiple cells (their nuclei) are lagging behind in maturation.

When one gives B12 to such a patient, cell maturation is suddenly accelerated and this process consumes a lot of potassium. Potassium is driven into the cells.

This can cause a sudden lowering of potassium in the serum. It can even lead to arrhythmias and Death. Before giving B12 for the first time in a such a patient , please check the potassium and if it is low, please replace it.