B12 After Bariatric Surgery, on PPIs or Metformin: Who Cannot Absorb It

After stomach surgery. Gastric bypass removes most of the acid-producing stomach and reroutes food past the part that makes intrinsic factor; the sleeve removes a large portion of the acid- and intrinsic-factor-producing lining. Either way, the main route is gone for good. B12 deficiency after bariatric surgery is expected rather than possible, lifelong supplementation is standard in every bariatric guideline, and injection is one of the accepted routes — alongside high-strength oral or sublingual products, which some patients absorb adequately by diffusion and some do not. Levels are checked at least annually for life.

Pernicious anemia. An autoimmune attack on the stomach lining destroys the cells that make intrinsic factor. It is the classic cause of B12 deficiency, more common after fifty and in people with other autoimmune conditions (thyroid disease especially), and it is the condition for which B12 injections were invented. Injection is the standard treatment and is lifelong. Diagnosis is by intrinsic-factor antibodies — which are specific but miss about half of cases — and the clinical picture.

Long-term acid suppression. Proton-pump inhibitors and, to a lesser degree, H2 blockers reduce the stomach acid that releases B12 from food. Absorption of supplemental B12, which is not protein-bound, is unaffected — so the deficiency develops slowly, over years, and is usually correctable with an oral supplement rather than an injection. Injection is reserved for the person who is symptomatic, severely low, or not responding to tablets.

Metformin. Metformin interferes with the calcium-dependent step at the ileal receptor, and long-term use lowers B12 in a substantial minority — a well-documented effect that the metformin hub covers in its own section, and that the ADA now recommends monitoring for. As with PPIs, oral supplementation usually corrects it; injection is for the person who does not respond.

Conditions of the ileum. Crohn's disease affecting the terminal ileum, surgical removal of it, celiac disease, bacterial overgrowth and some parasitic infections remove or impair the absorption site itself. Injection is the standard route where the ileum is gone or badly diseased.

Vegans and near-vegans. No absorption problem at all — the route works; there is simply no B12 in the diet. Oral supplementation is the answer, and injection is only for someone who has become severely deficient before starting it.

Older adults. Acid production and intrinsic factor both decline with age, and atrophic gastritis becomes common, so that a meaningful share of people over sixty have marginal B12 despite an adequate diet. Most are corrected orally; some — particularly those with neurological symptoms, covered on the nerve symptoms page — are better started with injection.

Two things. Whether you are low at all — serum B12, methylmalonic acid and homocysteine, on the baseline panel — because the majority of people asking for a B12 shot for energy are not, and the main page is honest about what a shot does for someone who is not deficient. And, if you are low, why: whether it is diet, a drug, a surgery, or an autoimmune process, because the cause determines both the route and how long treatment continues.

A person in one of the groups above who is low is a straightforward injection candidate, and the injection is doing exactly what it was designed for. A person outside them who is low needs the cause found. A person who is not low needs neither.