I've just been browsing the BNF (as you do) and came across an interesting bit.
One growing cause of a B12 deficiency is a dietary insufficiency caused by a vegan diet. If you are deficient and your doctor suspect it may be dietary then they might prescribe cyanocobalamin tablets. But the BNF includes this warning...
Currently available brands of the tablet may not be suitable for vegans.
WIth no clues as to which brands might be suitable!
So, if you are a vegan who is B12 deficient and your doc perscribes tablet then you may want to buy your own instead - amazon.co.uk/Health4All-Vit...