Shots or at least 1000 mcg per day orally
Although daily oral doses of 1000 - 2000 mcg B12 (NOT timed/slow release) has been properly updated textbook treatment for severe malabsorption (pernicious anemia or stomach surgery) for years, most medical people do not know it and believe shots are the only treatment. Even a person with pernicious anemia* (severe malabsorption due to lack of intrinsic factor) will absorb about 1% of the dose, which is enough for daily use and to rebuild stores over time.
Goldman: Cecil Textbook of Medicine, 21st Ed., W. B. Saunders Company, Page 1056
"Oral cobalamin, 1 to 2 mg/day [1000 to 2000 mcg per day], is the treatment of choice for most patients. This [oral] dose is as effective and possibly superior to a parenteral [shots] regimen in all causes of cobalamin deficiency because 1 to 2% of an oral dose is always absorbed by diffusion. For patients who may not be compliant [may not continue treatment on their own], intramuscular or subcutaneous cyanocobalamin can be given. One approach consists of injections of 1 mg of cyanocobalamin [the cheapest and least effective form of B12] once per week for 8 weeks, then once per month for life. More frequent injections often are used in hospitalized patients or patients with marked neuropsychiatric abnormalities, but no evidence of incremental benefit has been shown. When the weekly injections are completed, the patient or a family member or friend can be taught to give the monthly injections. Parenteral and high-dose oral regimens give prompt and equivalent hematologic and neurologic responses, but post-treatment serum cobalamin levels are significantly higher and post-treatment methylmalonic acid levels are significantly lower with the oral regimen. With either the parenteral or the oral regimens, the absolute requirement of lifetime therapy must be well understood by the patient and the patient's family. Because cobalamin is inexpensive and free of side effects, it is better to give too much than too little [If there is a symptom indicating reaction, it is a good thing]."
[bolding and brackets added by rose]
(Note: Individuals vary greatly in their abilities to store B12. A shot once per month will be fine for some, but not for others. Also, cyanocobalamin must be converted by the body for use, and a small percentage cannot convert or convert well. In those cases, a better form of B12 can be crucial. See B12 types)
*Pernicious Anemia is not an anemia, no matter who tells you it is. The anemia some people develop when B12 deficient is megaloblastic or macrocytic anemia. See Myths
Infants and Children:
When a mother is deficient in B12, the baby is born at a disadvantage. If the baby can absorb and use B12 normally, B12 stores will be increased slowly as fortified or B12-rich foods are introduced, but when B12 is needed the stores should be filled quickly.
If the baby cannot absorb or use B12 normally, rather than building stores those stores from the mother will be depleted, most likely by sometime in toddlerhood.
Some babies are born with defects that prevent their use of B12. Much more information will appear on this site, but in the meantime, for a doctor's information in cases of babies failure to thrive (or toddlers' health deteriorating as their B12 stores they were born with begin to run out):
From Pediatric Critical Care Medicine, Volume 6 • Number 4 • July 2005
Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies
Coma and respiratory failure in a child with severe vitamin B12 deficiency
"...Treatment with vitamin B12 (1 mg/day), folate (7.5 mg/day...."
(Note: 1 mg = 1000 mcg)
Some people simply need B12, but it is a good idea to cover all of the B vitamins just in case more are low. B complex will generally contain the standard 400 mcg folic acid (with which B12 works very closely), as well as other B vitamins in various doses.
Plenty of potassium-rich foods will usually take care of the short term heavy draw on potassium (for those who have been deficient and begin getting the B12 they need to rebuild stores).
During repairs, iron stores are also likely to be heavily drawn on. And, one of the common reasons for B12 malabsorption also affects iron absorption to a lesser degree. So a ferritin test is a very good idea for determining where a person stands, since it is not a good idea to take iron without knowing it is needed and the iron tests commonly ordered often are misleading.
More to come...