Those who suffer from megaloblastic anemia should get vitamin B-12 shots, according to the National Institutes of Health. This type of anemia happens when red blood cells are too large or too few or have other impairments that make it difficult for them to carry oxygen properly.
Doctors may also give vitamin B-12 shots to people who have pernicious anemia, according to NIH. This is a type of autoimmune disease that prevents the body from making intrinsic factor, which is necessary to absorb vitamin B-12. This is a problem seen in older adults, whose stomachs may not produce enough hydrochloric acid to support intrinsic factor.
People who have had bariatric surgery for weight loss may also need shots of vitamin B-12, claims the NIH. Other people who can benefit from B-12 injections are people with celiac disease. These people cannot tolerate gluten and may have problems with malabsorption. Patients who suffer from Crohn's disease, an inflammatory disease of the bowel, may also have trouble absorbing B-12 from food or from supplements. Since B-12 is found almost exclusively in animal products, strict vegetarians may benefit from injections of vitamin B-12. Vitamin B-12 shots may also help people with pancreatic or bowel cancer, according to MedicineNet.