Vitamin B12 is very important for proper functioning of the body. Your body requires this vitamin to maintain healthy nervous system (that's why it's often called an anti-stress vitamin), produce red blood cells, and even to replicate the DNA. Without this vitamin you'll get into trouble very soon. Because of so important role of this vitamin, the deficiency of it can't stay unnoticed for a long time.
Vitamin B12 deficiency symptoms can be divided into three groups: hematological, nervous, and gastrointestinal. As this vitamin is involved into red blood cell formation, the lack of it causes a special form of anemia, which is called pernicious anemia. You can search this name in the Internet to learn more about it.
Deficiency of this vitamin causes a lot of neurological problems such as nervousness. As I already mentioned, this vitamin is often called anti-stress vitamin because of its ability to cure nervousness and stress caused by its deficiency. Other common neurological problems caused by it are memory loss, confusion, disorientation, and even hallucinations. Besides, the cognitive decline in old people, which is traditionally associated with their age, is often caused by vitamin B12 deficiency, because its level in the body decreases with age.
There are also some gastrointestinal problems that may be caused by deficiency of this vitamin. They include diarrhea, loss of appetite, sore mouth, and nausea. If you experience symptoms mentioned above, you should consult your doctor as fast as possible.
We get this vitamin from foods of animal origin such as meat, eggs, and dairy. If you are a strict vegetarian who doesn't eat eggs and milk, you risk becoming vitamin B12 deficient. You should take vitamin B12 supplement in this case. People who eat balanced diet don't risk anything.
To illustrate this, I will tell you a story of my friend. Once he decided to become a vegan, because he thought that people should not eat animals. He also refused to eat eggs and milk. At the beginning he felt great, but after a period of time his health started to decline, because his vegan diet was poorly planned. He had become nervous and forgetful. His skin had become pale and unhealthy. Once I suggested him to visit a doctor. He agreed. The doctor told him to measure vitamin levels in blood. The blood test showed that he had low vitamin B12 level, as well as low iron level. After that he started to take vitamin supplements and his health normalized. Later he abandoned the vegan diet he used to follow, because his philosophical beliefs changed. He started eating animal products as all normal people do, and his health has become even better.