A Cochrane review of oral versus intramuscular (IM) vitamin B12 for vitamin B12 deficiency states that low quality evidence shows oral and IM vitamin B12 having similar effects in terms of normalising serum vitamin B12 levels, but oral treatment costs less. It also states that no trial reported on clinical signs and symptoms of vitamin B12 deficiency. These findings concern people with deficiency and do not carry over to people without one.
Reviews and the meta-analysis
The Cochrane review states that three RCTs met its inclusion criteria. The trials randomised 153 participants (74 participants to oral vitamin B12 and 79 participants to IM vitamin B12), and treatment duration and follow-up ranged between three and four months. It adds that no trial reported on health-related quality of life or acceptability of the treatment scheme.
A 2025 systematic review and meta-analysis reported that no statistically significant differences were observed between oral, sublingual, and IM routes of administration (p = 0.270 for cobalamin levels and p = 0.485 for homocysteine levels). The abstract text used here does not state who was studied, the doses or the length.
A review of vitamin B12 deficiency in American Family Physician states that oral administration of high-dose vitamin B12 (1 to 2 mg daily) is as effective as intramuscular administration for correcting anemia and neurologic symptoms. The same review states that intramuscular therapy leads to more rapid improvement and should be considered in patients with severe deficiency or severe neurologic symptoms. The Cochrane finding above, that no trial reported on clinical signs and symptoms, is not reconciled here.
Randomized trials in people with deficiency
Each trial describes people with deficiency or low serum vitamin B12 values. Sizes and doses appear only where the summary states them.
| Trial | Group and size | Doses as stated | Authors' conclusion |
|---|---|---|---|
| OB12, BMJ Open 2020 | 283 patients aged 65 years or older with VB12 deficiency; oral (n=140) or IM (n=143) | IM: 1 mg on alternate days in weeks 1-2, 1 mg/week in weeks 3-8, 1 mg/month in weeks 9-52. Oral: 1 mg/day in weeks 1-8, 1 mg/week in weeks 9-52 | Oral was no less effective than IM at 8 weeks. Differences were found at week 52, and the probability that they were below the non-inferiority threshold was very low |
| Blood 1998 | People with cobalamin deficiency; size not stated | Oral: 2 mg cyanocobalamin daily. IM: 1 mg monthly | Oral was as effective as IM and may be superior |
| Clin Ther 2003 | Patients with megaloblastic anemia due to cobalamin deficiency; size not stated | Not stated | Oral was as effective as IM, better tolerated and less expensive |
| Am J Clin Nutr 2018 | Patients with low serum vitamin B-12 after Roux-en-Y gastric bypass (RYGB); size not stated | Oral compared with hydroxocobalamin injections; doses not stated | Efficacy of oral was similar; oral can be used as an alternative |
What the label says
The prescribing information for cyanocobalamin injection states that oral absorption is considered too undependable to rely on in patients with pernicious anemia or other conditions resulting in malabsorption of vitamin B12. For patients with normal intestinal absorption, it says that where the oral route is not deemed adequate, initial treatment similar to that for pernicious anemia may be indicated depending on the severity of the deficiency, and that chronic treatment should be with an oral B12 preparation. These are label statements for that product and its labeled populations.
Other forms described by NHLBI and the FDA
The NHLBI states that vitamin B12 medicine can be prescribed to take by mouth or as a nose spray or a shot. The Nascobal nasal spray label states that the recommended initial dose is one spray (500 mcg) administered in ONE nostril once weekly. Drugs@FDA lists Nascobal cyanocobalamin 0.5MG/INH gel, metered; nasal under NDA 019722, and the nasal gel listing is marked Discontinued.
The spray label and the gel listing are separate records, and the sources used here do not say how they relate or which group the nasal doses are for. This section reports no trial of nasal forms.
What this does not tell you
The trials in the table, the Cochrane review and the American Family Physician review concern people with deficiency or low serum values, so they say nothing about people without one; the meta-analysis summary does not say who was studied. Most summaries do not name the outcome, so "as effective" is each team's wording. Only one summary mentions tolerability, and none reports harms for either route. No source here says which route a person should use or whether to change it; a licensed provider decides. None of the sources used here reports on a compounded vial.