B12 is an essential vitamin that is necessary for a number of critical hormonal and metabolic functions. It's also believed to be one of the most common nutrient deficiencies in the world. There are a number of reasons why B12 deficiencies are so common.
B12 is derived from meat, fish, and poultry and not plant based foods. Very often, people who do not consume meats in their diets can be lacking adequate B12.
Overall, B12 injections are safe with very few known side effects or adverse reactions. Mild reactions like diarrhea or pain at the injection site can occur. Serious side effects, although extremely rare, include confusion, shortness of breath, headache, reddening of the skin or irregular heart rate.
B12 can negatively interact with certain medications like metformin, proton pump inhibitors and certain antibiotics.