Vitamin B12, or cobalamin, is a large, complex molecule involved in a variety of bodily processes, ranging from metabolism to brain chemistry to blood cell production. B12 is necessary for producing healthy red blood cells and platelet cells, according to the National Institutes of Health (NIH). A B12 deficiency can lead to megaloblastic anemia, a blood disorder that causes weakness, fatigue, trouble concentrating, irritability, headache, heart palpitations and shortness of breath.
People who consume few to no animal products (namely vegans and vegetarians) have a higher risk of developing a B12 deficiency. That said, eating foods fortified with vitamin B12, including whole grains, cereals and nutritional yeast, and also supplementing with the vitamin can reduce the risk of deficiency, per the NIH.
Note: I’m a vegetarian and I take a daily B12 supplement.