here is a good info page that might answer some more questions.
B12 Fact Sheet
the vitamin itself seems harmless and there are no chances of toxicity so you can't abuse the vitamin, but there is also the option of fortefied foods if you prefer not to take a supplement form.
perhaps try it in the tablet form for a couple of week - the addition of folate is always good for women as we tend not to hold iron as well as men, especially around mensturation.
if you are anaemic which is probably the reason your doctor would have suggested B12, you can get an idea for it yourself at home. of course, paleness and tiredness, pale lips are also good indicators.
look in the mirror and gently pull down your lower eye lid so you can see inside your lower eye lid essentially. if you are deficient in vitamin b the inner eye lid will be a pale pink as opposed to a more reddish colour.
another way is to have your palm face up towards you - either hand - and with your other hand press down with your thumb on the fleshyness just above your wrist on your little finger side. when you release your thumb (you need to press quite hard) if it takes a while for the whiteness to go and the natural colour of your skin return, you are most likely iron deficient. it should spring back immediately as the surrounding colour.
if you aren't taking any other type of supplement how about looking at an all round womens vitamin? they will have all the goodies you need, including the B vitamins.
info on anaemia
God - i am the anaemia queen!
i suffered from it for a long time and still do. problem is i forget to take my multivitamins