B12 Injection vs Tablets: When the Route Matters and When It Does Not

The active pathway. B12 in food is bound to protein. Stomach acid releases it, it binds intrinsic factor made by the stomach lining, and that complex is absorbed at the end of the small intestine, the terminal ileum. This route is efficient but handles only a limited amount at a time. It is the one that fails in autoimmune gastritis, after stomach surgery and where the terminal ileum is diseased or removed.

The passive pathway. Separately, a tiny fraction of free B12 crosses the intestinal wall by diffusion, with no intrinsic factor and no receptor. The Cochrane review of oral versus injected B12 notes that this passive absorption is unaffected in people with pernicious anemia or after gastric surgery. That is why large-amount oral B12 can work even when the active route has gone.

The absorption groups page works through who loses which route, from long-term acid suppression and metformin to bariatric surgery.

This has been studied directly, which is more than most route comparisons can say.

A 2018 Cochrane review pooled three randomized trials comparing oral with intramuscular B12. It found low-quality evidence that the two have similar effects on normalizing blood B12 levels, that oral treatment costs less, and very low-quality evidence that oral B12 is as safe. The trials were small, and none reported on symptoms, quality of life or how acceptable people found each route. A 2024 network meta-analysis reached a similar conclusion: oral, intramuscular and sublingual B12 all raised B12 levels, without significant differences between them.

Two qualifications follow. The comparisons are about correcting a blood level, with little data on nerve outcomes. And guidelines have not all drawn the same conclusion from them, which is why the next section matters.

The UK's 2024 NICE guideline on vitamin B12 deficiency sets out the situations clearly:

• Autoimmune gastritis (pernicious anemia), total gastrectomy or complete terminal ileal resection. NICE recommends lifelong intramuscular B12. This is where NICE is more cautious than the trial summary above: passive absorption exists, but NICE still prefers injections here.
• Other malabsorption, such as celiac disease, partial gastrectomy or some bariatric procedures: NICE says to consider intramuscular rather than oral replacement, and if oral is used, a high oral amount.
• Nerve symptoms. NICE says not to delay treatment while waiting for test results when there are neurological symptoms, and for dietary deficiency it suggests considering injections where a condition could deteriorate quickly, such as ataxia. Nerve symptoms explains why speed matters: nerve damage present for long enough may not fully reverse.
• Doubtful adherence. Where daily tablets are unlikely to be taken, for example with cognitive impairment or frailty, a periodic injection is more reliable than a tablet not taken.
• When oral has been tried and levels have not responded, which is a straightforward reason to change route.

NICE suggests either route, by clinical judgment and preference, for deficiency caused by a medicine such as metformin or acid suppression; oral B12 for dietary deficiency, as in vegans; and oral rather than injected B12 where the cause is unclear and malabsorption is not suspected, with a review of the response at follow-up.

Which form? Cyanocobalamin and hydroxocobalamin are the standard forms. NICE advises that an over-the-counter supplement should contain cyanocobalamin, methylcobalamin or adenosylcobalamin, because some products do not contain enough, or the right type, to be effective. Cyanocobalamin vs methylcobalamin covers the marketing claims.

How long for? It depends on the cause. Autoimmune gastritis and surgical causes mean lifelong treatment. A medicine-related deficiency may be reviewed if the medicine can be stopped or changed.

Can you take too much? B12 is water-soluble, and excess is generally excreted rather than stored to any useful effect, which is why extra B12 in someone with adequate levels is unlikely to change how they feel.

Whichever route is used, the more important question is why B12 was low. Autoimmune gastritis needs lifelong treatment. Long-term acid suppression or metformin may be reviewable. Malabsorption points to something worth investigating. A dietary cause is simple but worth confirming. Supplementing before testing can also mask a deficiency, so testing comes first where possible. Treating the number without asking why leaves the actual condition in place; why a normal panel does not close the question covers the wider point.

The B12 injection at ACT 2 is $149 a month, covering the clinician's care and the medication. ACT 2 does not sell B12 tablets; they are priced by whoever sells them. Lab work is priced separately.

• When B12 is not low. Additional B12 in someone with adequate levels is largely excreted, and the energizing feeling some people report after a shot is not well supported.
• When fatigue is the reason and nothing has been tested. Thyroid, iron, vitamin D, blood sugar and sleep produce the same complaint; what your tiredness pattern says is a good start.
• When nerve symptoms appear suddenly or are severe. That needs in-person assessment, not a choice of route.