Vitamin B12 shots do not increase weight loss or boost energy in most individuals, according to Healthline. However, someone with a vitamin B12 deficiency that caused megaloblastic anemia would feel better by taking a prescribed, high-dosage shot of vitamin B12 because this type of anemia causes fatigue. For someone without a deficiency, there is no known risk for receiving vitamin B12 shots, but there is also no benefit.
Sometimes people with type 2 diabetes take metformin as part of a weight loss program, explains the Mayo Clinic. One side effect of metformin is vitamin B12 deficiency, so patients taking the drug should talk to their doctors about how to avoid developing low vitamin B12.
People who have a higher risk of vitamin B12 deficiency include vegetarians, people with particular digestive diseases, people who have had gastrointestinal surgery, people who have HIV and the elderly, notes Healthline. People who have low stomach acidity, such as the elderly or people who have had gastrointestinal surgery, have a reduced capacity for absorbing vitamin B12. For these people, consuming supplements or vitamin B12 fortified foods may be beneficial. Supplementation is also essential for vegans and vegetarians because animal-based foods contain the largest amounts of vitamin B12.
Patients who receive vitamin B12 shots should disclose this fact to their doctors because it can interfere with some prescription medications, cautions the Mayo Clinic.