The only natural dietary sources of vitamin B12 are animal products including organ meats, fish, dairy, beef, pork and eggs. In order for a vegetarian, and especially a vegan, to get the recommended amount of vitamin B12 in their diet therefore it is necessary to use enriched foods or take good multi-vitamin supplements with vitamin B12 or have vitamin B12 injections.
Due to the fact that vitamin B12 is only found naturally in animal products vegans is the people group most susceptible to vitamin B12 deficiency. This deficiency can lead to "loss of energy, tingling, numbness, reduced sensitivity to pain or pressure, blurred vision, abnormal gait, sore tongue, poor memory, confusion, hallucinations, and personality changes." (http://www.vegansociety.com/html/food/nutrition/b12/)
In order to prevent vitamin B12 deficiency, vegans should look for products that are enriched with vitamin B12. A number of cereals and plant milks may be enriched with vitamin B12. When choosing your enriched foods check the labels to see how much vitamin B12 you will be getting from each serving and work out how much you will need to eat daily in order to get the required amount of 2.4 to 2.8 micrograms.
It used to be believed that there was vitamin B12 in certain plant foods, however, today views are changing and generally it is believed that what B12 is in plant food is not able to be used by our human bodies and so is therefore not a good source of vitamin B12.
In order to ensure that you receive enough vitamin B12 when following a vegetarian or vegan diet it is therefore recommended that apart from using foods that have been enriched with vitamin B12 that you also take vitamin B12 supplements.
If you are a strict vegan or vegetarian you should also have a test for vitamin B12 done regularly to ensure that you are not putting yourself at risk of a deficiency. The best test you can do is a test for methylmalonic acid, other tests for vitamin B12 may lead to incorrect readings due to B12 analogues found in plant foods, or high folate levels masking anemia in blood count testing.
In order to ensure that you get a sufficient supply of vitamin B12 in your vegetarian or vegan diet it is necessary to use enriched foods, take vitamin B12 supplements and get tested regularly using the methylmalonic acid testing method to ensure that you do not suffer from a deficiency of B12, leading to loss of energy, blurred vision, abnormal gait, confusion, hallucinations and various other symptoms.