Do not take B12 with food.... I don't care who told you otherwise, as they are not well informed.

Drugs in microgram quantities like levothyroxine, and digoxin and other things are not well or absorbed at all in the presence of food. This has been well documented, but not for supplements yet. It could be 50 more years of waiting and failing.

B12 is absorbed two ways. 1) with intrinsic factor which is designed to grab small 1-4mcg amounts of B12 out of protein you eat. If this fails, and intrinsic factor will not be secreted in low acid environments or when damaged by autoimmune attack, the only way B12 can be absorbed is passively in the small intestine instead. Food absorbs it like sponge and keeps it from getting absorbed. 2) passive absorption of B12 was not believed until very recently... in some studies. But many doctors don't read those.

People very low in B12 --- if you have your test results, please post here. Very low test results, really should have 5 mg of methylcobalamin daily on an empty stomach. Borderline low patients can probably get by with 1mg of this daily.
B12 is the only B I know of at this time that has special requirements. Most other vitamins can be taken with food.

My B12 thread:
http://neurotalk.psychcentral.com/thread85103.html
You will see from the medical sites and medical studies, on that thread that this is not a simple situation. Trust me, empty stomach for B12. Best taken alone, for that reason. B-complexes with B12 in them, the B's can upset the stomach.
So they are typically taken with food. Methyl B12 alone only costs pennies a day and is not a hardship.

Iron on the other hand can cause significant upset stomachs and constipation. That is because most is not absorbed because the body limits iron absorption because of its toxicity.

When people are low in B12 it is because of a PROBLEM with it. I assume people with problems most likely have low acid or intrinsic failure or they wouldn't be low. Long term vegans get low because of diet. Some drugs deplete B12, also. But since B12 has no upper limit of safety and is easily tolerated and very inexpensive, you don't need to worry about like you should for other types of supplements.