Reasoning GP · Algorithm · 1-page summary
Vitamin B12 deficiency in adults
For: people over 16 with suspected or confirmed B12 deficiency, including pregnancy, metformin, vegan diet and nitrous oxide. Not for: children (paediatric advice). Low folate → the low folate algorithm; raised MCV with normal B12 → the macrocytosis algorithm.
Core rule: neurological features → IM hydroxocobalamin now, without waiting for results or MRI. A normal Hb or MCV does not exclude deficiency. Replace B12 before folate, and never stop injections in pernicious anaemia.
0Stop — act today — before anything else
- Suspected subacute combined degeneration (SACD): progressive leg weakness or stiffness, loss of vibration or proprioception, positive Romberg, sphincter symptoms → IM hydroxocobalamin now + Same-day medical/neurological assessment.
- Nitrous oxide use + neurological symptoms → IM hydroxocobalamin without waiting for results + Same-day assessment. Serum B12 may be normal: test homocysteine or MMA.
- Acute confusion or psychosis → Same-day medical assessment for delirium and other causes. Never blame B12 alone.
- Severe symptomatic megaloblastic anaemia or pancytopenia → Same-day hospital. Transfusion is an individual decision.
1Triage by result — total B12 (NG239), with symptoms and FBC
| Result | Action |
|---|---|
| <180 ng/L | Deficient → treat and find the cause. |
| 180–350 ng/L | Indeterminate (active B12 25–70 pmol/L). Symptoms → consider MMA; consider starting B12 while awaiting it. |
| >350 ng/L | Less likely; not excluded if neurological symptoms, recent N₂O or pregnancy. |
Test: pregnancy → active B12; combined pill → consider active B12; N₂O → homocysteine or MMA.
2Screen — serious causes and complications
- Breastfed infant of a severely deficient mother, neurological deterioration → Same-day paediatric / obstetric.
- B12 <180 ng/L + peripheral neuropathy → start IM now; Same-day assessment if progressive, severe or uncertain.
- Autoimmune gastritis + new or worsening dyspepsia, nausea or vomiting → consider gastroscopy; NG12 suspected-cancer pathway where criteria are met.
- Suspected MDS or leukaemia → Urgent haematology advice; Same-day if film or count suggests acute leukaemia.
3Assess
☐ FBC + film, reticulocytes ☐ Total or active B12, folate ☐ Anti-intrinsic factor antibody if autoimmune gastritis suspected ☐ LFTs, U&E; TFTs if indicated; anti-tTG + total IgA if GI features ☐ Diet, medicines, gut surgery, N₂O (ask non-judgementally), autoimmune and family history
Not routine: surveillance gastroscopy; annual anti-TPO, anti-GAD or cortisol; serum B12 to monitor regular IM treatment.
4Classify — find the cause
- Autoimmune gastritis (pernicious anaemia) — anti-IF positive supports; negative → consider parietal cell antibody, gastrin or biopsy. Lifelong IM.
- Diet — vegan or strict vegetarian → oral B12 + fortified foods.
- Medicines — metformin, long-term PPI → replace; continue metformin.
- Gut surgery — total gastrectomy, complete ileal resection → lifelong IM.
- Nitrous oxide — B12 may look normal → homocysteine/MMA; stop N₂O.
- Uncertain / poor response → reassess cause; raise oral dose or switch to IM.
5Treat today — doses: NICE/SPC, BNF, local formulary
- No neurological features (pernicious anaemia, malabsorption): hydroxocobalamin 1 mg IM 3 times a week for 2 weeks, then every 2–3 months.
- Neurological features: 1 mg IM alternate days until no further improvement, then every 2 months.
- Oral cyanocobalamin: diet 50–150 micrograms daily between meals; malabsorption treated orally ≥1 mg daily; metformin 1 mg daily or IM by judgement and preference.
- Folate deficiency too: start B12 first or at the same time. Never folic acid alone.
6Refer routinely — and follow up
- Review at about 3 months (earlier if severe): symptoms, adherence, response.
- Neurology: neuropathy not better by 3 months; disproportionate cognitive decline.
- Gastroenterology: suspected malabsorption or coeliac disease; confirming autoimmune gastritis.
- Metformin: check B12 if symptoms; consider periodic monitoring if other risk factors (MHRA).
7Never reassure by
- A normal Hb or MCV — nerve damage can occur without anaemia.
- A normal B12 after nitrous oxide or with strong neurological features.
- A rising Hb on folic acid alone — SACD can still progress.
- A negative anti-intrinsic factor antibody or a normal MRI spine.
- Normal bloods as a reason to stop injections in pernicious anaemia.
- A low B12 as the explanation for acute confusion.
8Safety-net — say it, then document it
Same day “Contact us the same day if you get new numbness or tingling in your hands or feet, weakness or stiffness in your legs, unsteadiness or falls, problems controlling your bladder or bowels, or sudden confusion — especially after using nitrous oxide.”
Pernicious anaemia: “The injections are for life — never stop them. Come back if symptoms return before the next one.” Book the 3-month review with a date.
✓Safe to manage in primary care only if — all five, documented
No block 0–2 row applies; no progressive neurology
Result read with symptoms, FBC and the right test
Cause sought: diet, medicines, anti-IF, surgery, N₂O
Route matched to cause; B12 before folate
Safety-net given; 3-month review booked
Summary of the full Reasoning GP vitamin B12 deficiency algorithm, which holds the detail, physiology, test interpretation and evidence. Sources: NICE NG239 (2024); MHRA Drug Safety Update (June 2022); BSH cobalamin and folate (2014); BSG gastric adenocarcinoma risk (2019); NICE NG12 (updated April 2026). Clinical reference — check doses in the current BNF and follow local protocols.