We found a Cochrane review of oral vs IM vitamin B12 administration. There are few direct trials but it does appear that high dose oral B12, either at the dose of 2,000 or 1,000 micrograms (the latter given daily initially, then weekly, then monthly), is as effective as IM in obtaining haematological and neurological response (Vidal-Alaball J, et al. Cochrane Database Syst Rev 2005; 3: CD004655).
It would be worth practices reading the literature and then discussing with patients which option they would prefer until more hard evidence is available.
Unfortunately this is made a little more complicated by the lack of suitable products to prescribe, although oral B12 in a 1,000 microgram preparation is available in 'health food shops' for about £7 per 100.
One other common cause of deficiency is metformin use.
Dr Francina Schreuder, Foundation Year 2 Doctor, Kent.