B₁₂ is an essential vitamin that it is "hard to get" by people - not exclusively vegans.

We only need traces of B12. It is also known as methylcobalamin, cyanocobalamin, hydroxocobalamin and adenosylcobalamin. 

Michael Greger M.D. on his Nutrition Facts website has plenty of info about B12. The reports that Michael Greger cites favour a type of B12 called cyanocobalamin. This differs from most other online reports which tend to favour another type called methylcobalamin! I sit on the fence and supplement with some of each!

B₁₂ is bacteria, usually found in healthy soil. Consequently, animals eat it and animal eaters can sometimes obtain enough from animal sources although this is getting less likely as more animals are not raised outside but in massive barns/sheds. To maintain the animal industry's marketing myth that B₁₂ only comes from animals many cows are now fed B₁₂. Unfortunately, more and more vegetables are not even grown in soil or are grown using chemicals that destroy the sources of B₁₂. It is only found in/on plants as a "contaminant".

The issue of B₁₂ is a complex one. Some reports state that it is destroyed by heat and that most people in western society are deficient. An estimated 40-70% of there UK population has a deficiency. This is a page showing seven signs of deficiency.

The UK Vegan Society (see this article) and the Physicians Committee for Responsible Medicine, among others, recommend that every vegan take foods fortified with B₁₂ and/or supplements.

This page from veganhealth.org has all the info you could possibly need to know about B₁₂

Viva has a comprehensive B₁₂ fact sheet.

This comprehensive article from Whole Health Chicago explains why B₁₂ is still North America's number one missed diagnosis.

RDA - (recommended daily allowance) of B₁₂ varies with age and circumstances.

A deficiency can be very problematic. It is likely to be a problem with intestinal flora caused, often by antibiotics, that hamper the bodies own production of B₁₂ - from Vibrancy UK article

Deficiencies have also been shown to harm cognitive function, especially in old people - whatever their diet. It has been found that people who were considered to have the symptoms of dementia or Alzheimer's improved once they were given B₁₂   Too much has a link to acne but is generally just considered expensive pee as normally too much is mere waste.

Don't forget that the NHS B12 test cannot distinguish between useless and useful B12. Ergo it can give false positives!

On this NHS page it states this: 
"A particular drawback of testing vitamin B12 levels is that the current widely-used blood test only measures the total amount of vitamin B12 in your blood. This means it measures forms of vitamin B12 that are "active" and can be used by your body, as well as the "inactive" forms, which can't. If a significant amount of the vitamin B12 in your blood is "inactive", a blood test may show that you have normal B12 levels, even though your body can't use much of it. There are some types of blood test that may help determine if the vitamin B12 in your blood can be used by your body, but these aren't yet widely available."

There are several ways to take B12 - as an oral spray or as tablets (which you usually need to chew). Don't be alarmed that the tablets contain way over the RDA - this is due to the body's inability to absorb B12 easily. Nutritional yeast flakes - maybe six teaspoonfuls a day - probably provides enough as well. Nutritional yeast is not the same as yeast used for baking bread. These flakes, which have a nutty cheese-like taste, are great for cooking - ideal for sprinkling on top of many dishes; stews, soups, flans, pizzas etc.

I have only ever seen a couple of warnings about B12 usage: 
  • If you suffer from Leber's disease then taking B12 supplements can seriously damage the optic nerve which may lead to blindness.
  • B12 can also unmask the symptoms of Polycythemia Vera.

B12 is in (human) urine; it is not just used in cosmetics. Urine (the mid-stream morning urine is said to be best) has been used as an alternative medical treatment for thousands of years. This article explains it.  For the sceptics; this method of healing is today even taught on medical universities, mainly in Japan and Mexico. In India, urine therapy has been used for thousands of years by yogis, and it is taken as a holy ritual, to drink morning glass of urine. People may have a lot of prejudices about our urine, but it is not a waste product but our own blood plasma and contains many nutrients, including vitamins, minerals, proteins, antibodies and other beneficial ingredients. It is not recommended for animal eaters because of the higher urea content.

There are many books about urine therapy including this one and plenty of information online - the wikipedia entry is a good starting point!

Here's an intriguing "incentive" not to become deficient!