If you are following or considering a vegan or vegetarian eating plan, I sincerely recommend that you read the article at this link on the importance of vitamin B12. Vitamin B12: Your Key Facts (The Vegan Society)

There are no plant-based sources of Vitamin B12, an essential nutrient. A daily or weekly supplement is the best way to avoid the very serious consequences of Vitamin B12 deficiency. Some vegan food products, such as nutritional yeast, may be fortified with Vitamin B12, but I prefer to be sure I’m covered by taking a daily supplement.

Vitamin B12 the only vitamin that cannot be sourced from a whole-food plant-based diet. Most people in developed countries get enough from the consumption of meat or foods with animal sources, but a high percentage of deficiency disorders have been observed in those who do not consume meat and who do not take a Vitamin B12 supplement. Deficiency symptoms may take five years or more to develop in adults.

Vitamin B12 was discovered as a result of studies related to pernicious anemia, an autoimmune disorder in which the blood has a lower than normal number of red blood cells.  The ability to absorb the vitamin declines with age, especially in people over 60 years old. The diets of vegetarians and vegans may not provide sufficient B12 unless a dietary supplement is consumed.

Symptoms of vitamin B12 deficiency may include limb neuropathy, tiredness, weakness, lightheadedness, headache, breathlessness, loss of appetite, pins and needles sensations, changes in mobility, severe joint pain, muscle weakness, memory problems, decreased level of consciousness, brain fog, and others. These symptoms have been observed in individuals with B12 levels only slightly lower than normal. 

Source: Wikipedia