Injectable vitamin B is used to treat anemia, chronic fatigue, and pernicious anemia.  Pernicious anemia is a byproduct of the body's inability to absorb vitamin B from food.  It can therefore be reliably treated only by the administration of injectable vitamin B.  It has also been used to treat many other conditions, with varying levels of success.

Injectable vitamin B has a list of side effects, of both the common, non-serious type and of the uncommon, potentially fatal type.  The common side effects are pain and a warm sensation at the injection site, mild diarrhea, upset stomach, a feeling of nausea, headache, joint pain, and a feeling, or sense of, being swollen over the entire body.

Injectable vitamin B can be quite costly, as each injection must be administered at a doctor's office and the patient needs to be monitored for a time afterwards to look for and possibly treat things like severe chest pain and difficulty breathing, which are uncommon but possible side effects of the medication.  In contrast, Vitamin B pills are available at most pharmacies, can be self-administered, and generally cost under $20.00 USD for a year's supply.  These pills can be used by people with pernicious anemia; however, since vitamin b can build up in the bloodstream, care must be taken to make sure that the levels do not become toxic.

There are people who should not take injectable vitamin B.  Among these people are women who are pregnant or lactating, because the levels of vitamin b may affect the developing infant.  Other people who should not, or who should never, take Injectable vitamin B are people who have Leber's disease, which is a type of optic neuropathy leading to blindness that affects young men, have kidney disease, liver disease, iron deficiency, folic acid deficiency, are receiving any treatment or taking any medication that affects the bone marrow or who have an allergy to cobalt, or any other food, dye, or preservative.  As one can see, this list can be quite long and inclusive.   These people would be better served by using Vitamin B pills instead of injectable vitamin B as the amount of vitamin b absorbed by the body through a pill is less at once.  Therefore, while therapy would take longer to be effective, it would be safer for these people.