Quotes:
"Vitamin B12 deficiency anaemia is usually treated with injections of vitamin B12, in a form called hydroxocobalamin.
At first, you'll have these injections every other day for two weeks, or until your symptoms have stopped improving. Your GP or nurse will give the injections.
After this initial period, your treatment will depend on whether the cause of your vitamin B12 deficiency is related to your diet. The most common cause of vitamin B12 deficiency in the UK is pernicious anaemia, which isn't related to your diet"
and . . .
"If your vitamin B12 deficiency isn't caused by a lack of vitamin B12 in your diet, you'll usually need to have an injection of hydroxocobalamin every three months for the rest of your life"
Source:
http://www.nhs.uk/Conditions/Anaemia-vitamin-B12-and-folate-deficiency/Pages/Treatment.aspx