You should beware of B12 deficiency if you are over 50, suffer from Coeliac disease, do not eat packaged breakfast cereal with added vitamins, or you take sodium bicarbonate.
Symptoms may just be tiredness and fatigue, which may be mistaken for the normal symptoms of old age.
I personally tick all those boxes, but the health care professionals seemed unaware.
The U.S. Institute of Medicine recommends that everyone over 50 should obtain most of their vitamin B12 from vitamin supplements or fortified foods
B12 supplements have very low toxicity.
"Vitamin B12 deficiency is characterized by megaloblastic anemia, fatigue, weakness, constipation, loss of appetite, and weight loss. Neurological changes, such as numbness and tingling in the hands and feet, can also occur. Additional symptoms of vitamin B12 deficiency include difficulty maintaining balance, depression, confusion, dementia, poor memory, and soreness of the mouth or tongue. The neurological symptoms of vitamin B12 deficiency can occur without anemia, so early diagnosis and intervention is important to avoid irreversible damage"
"Vitamin B12, bound to protein in food, is released by the activity of hydrochloric acid and gastric protease in the stomach"
"fortified breakfast cereals are a readily available source of vitamin B12"
"Some people ‐ particularly older adults, those with pernicious anemia, and those with reduced levels of stomach acidity (hypochlorhydria or achlorhydria) or intestinal disorders ‐ have difficulty absorbing vitamin B12 from food and, in some cases, oral supplements. As a result, vitamin B12 deficiency is common, affecting between 1.5% and 15% of the general population"
"Individuals with atrophic gastritis are unable to absorb the vitamin B12 that is naturally present in food. Most, however, can absorb the synthetic vitamin B12 added to fortified foods and dietary supplements. As a result, the IOM recommends that adults older than 50 years obtain most of their vitamin B12 from vitamin supplements or fortified foods. However, some elderly patients with atrophic gastritis require doses much higher than the RDA to avoid subclinical deficiency"
"Individuals with stomach and small intestine disorders, such as celiac disease and Crohn's disease, may be unable to absorb enough vitamin B12 from food to maintain healthy body stores"
"The IOM did not establish a UL for vitamin B12 because of its low potential for toxicity"
Here is an Annotated Version of the NIH document. I have highlighted the lines I consider most important in yellow. The link to the original document is above.