Vitamin B12 Replacement 💉
Vitamin B12 replacement in Ireland: IM hydroxocobalamin dosing from the HPRA SmPC, when to use oral B12, folate, and red flags.
Vitamin B12 prescribing
Doses from the HPRA SmPC
| Drug | Dose | Frequency | Route | Notes |
|---|---|---|---|---|
| IM hydroxocobalamin | ||||
| Hydroxocobalamin 1 mg/ml: loading, no neuro signs | 250–1000 micrograms | Alternate days for 1–2 weeks, then 250 micrograms weekly until FBC normal | IM | Bloods before the first dose. Most wards give the full 1 mg ampoule. |
| Hydroxocobalamin 1 mg/ml: loading, neuro signs | 1 mg | Alternate days, then maintenance per local protocol | IM | Take bloods, then start without waiting for results. Discuss with haematology or neurology. |
| Hydroxocobalamin 1 mg/ml: maintenance | 1 mg | Every 2–3 months | IM | Lifelong for autoimmune gastritis, total gastrectomy, terminal ileal resection. |
| Oral | ||||
| Cyanocobalamin | 1 mg | OD | PO | No neuro signs. Use if malabsorption possible, pregnancy, or breastfeeding. Check local stock. |
IM hydroxocobalamin
Hydroxocobalamin 1 mg/ml: loading, no neuro signs
IMDose:250–1000 micrograms
Frequency:Alternate days for 1–2 weeks, then 250 micrograms weekly until FBC normal
Notes: Bloods before the first dose. Most wards give the full 1 mg ampoule.
Hydroxocobalamin 1 mg/ml: loading, neuro signs
IMDose:1 mg
Frequency:Alternate days, then maintenance per local protocol
Notes: Take bloods, then start without waiting for results. Discuss with haematology or neurology.
Hydroxocobalamin 1 mg/ml: maintenance
IMDose:1 mg
Frequency:Every 2–3 months
Notes: Lifelong for autoimmune gastritis, total gastrectomy, terminal ileal resection.
Oral
Cyanocobalamin
PODose:1 mg
Frequency:OD
Notes: No neuro signs. Use if malabsorption possible, pregnancy, or breastfeeding. Check local stock.
SmPCs (medicines.ie / HPRA)
📍 Guidance source
- There is no national HSE guideline for B12 deficiency. IM dosing here follows the Irish (HPRA) SmPC for hydroxocobalamin 1 mg/ml.
- Check your hospital or lab guidance. Reference ranges and preferred regimens vary between Irish sites.
🚨 Red flags
- Neurological signs (paraesthesia, loss of proprioception or vibration sense, ataxia, falls, cognitive change, visual loss): send bloods, then start IM hydroxocobalamin straight away. Do not wait for results. Discuss with haematology or neurology.
- Suspected subacute combined degeneration of the cord can become permanent if treatment is delayed.
- Severe symptomatic anaemia or heart failure: senior review. If transfusion is needed, give it slowly with close fluid monitoring, as megaloblastic anaemia carries a high risk of circulatory overload.
- Pancytopenia, haemolysis, or a blood film that does not fit: discuss with haematology before assuming B12 deficiency.
- Recreational nitrous oxide use: B12 levels can look normal. Check homocysteine or MMA and treat if clinically suspected.
📋 Before you treat
- Take bloods before the first dose: FBC, film, reticulocytes, B12, folate, ferritin, U&E. Treatment makes later B12 levels uninterpretable.
- Interpret B12 against your local laboratory reference range. A normal Hb or MCV does not rule out deficiency.
- Look for the cause: diet (vegan, poor intake), autoimmune gastritis (pernicious anaemia), gastrectomy, bariatric surgery, terminal ileal resection or Crohn's, coeliac disease, nitrous oxide.
- Drugs linked to low B12: metformin, PPIs, H2-receptor antagonists, colchicine, and some anticonvulsants (phenobarbital, primidone, topiramate, pregabalin).
- If the cause is unclear, send anti-intrinsic factor antibodies and coeliac serology. A negative anti-IF test does not exclude autoimmune gastritis.
⚠️ Folate: B12 always comes first
- Never start folic acid until B12 deficiency has been excluded or B12 replacement has started. Folic acid alone can correct the blood count while subacute combined degeneration of the cord progresses, sometimes permanently.
- Combined B12 and folate deficiency: give the first B12 dose at least 24 hours before starting folic acid (Nottinghamshire APC).
- B12 result still pending: do not start folic acid empirically. If severe megaloblastic anaemia means treatment cannot wait, discuss with haematology before giving both.
- Once B12 is under way: folic acid 5 mg once daily, usually for 4 months.
- Check the drug chart for folic acid already prescribed (including in combination products) when B12 deficiency is found.
⚠️ Other cautions
- Hypokalaemia can occur in the first days of treating severe megaloblastic anaemia (SmPC). Check potassium early and replace if needed.
- Reactive thrombocytosis can occur in the first weeks. Check the platelet count.
- Hydroxocobalamin contains cobalt. Use caution with a known cobalt allergy. It can turn urine pink or red.
💉 IM hydroxocobalamin (HPRA SmPC)
- Product: hydroxocobalamin 1 mg/ml solution for injection, 1 ml ampoule. IM only.
- Without neurological involvement: 250–1000 micrograms IM on alternate days for 1–2 weeks, then 250 micrograms weekly until the blood count is normal. Most wards give the full 1 mg ampoule for each loading dose.
- With neurological involvement: 1 mg IM on alternate days, then switch to maintenance per your local protocol.
- Maintenance: 1 mg IM every 2–3 months.
- Lifelong IM treatment for irreversible causes: autoimmune gastritis, total gastrectomy, terminal ileal resection.
- Document the loading plan and maintenance date on the discharge letter so the GP can continue it.
💊 Oral B12
- Reasonable for dietary deficiency, medicine-induced deficiency, or unclear cause without suspected malabsorption, if there are no neurological symptoms.
- Use oral cyanocobalamin 1 mg daily if malabsorption is possible, or in pregnancy or breastfeeding. Low-dose products are only suitable for purely dietary deficiency.
- Use IM if there are neurological symptoms, anaemia that needs a quick response, adherence concerns (frailty, delirium, cognitive impairment), or an irreversible cause.
- Check with pharmacy which oral B12 product your hospital stocks.
📅 Follow-up
- If anaemic, a reticulocyte rise should appear within about a week of IM loading. No response: question the diagnosis and check folate and iron.
- Iron deficiency can be unmasked as the marrow recovers. Recheck ferritin if Hb stalls.
- B12 levels are not useful once IM treatment has started. Judge response by symptoms and FBC.
- Ask the GP to review symptoms and FBC after the loading course, and to continue maintenance.
Related NICE guidance
NICENICE NG239 — Vitamin B12 deficiency in over 16s: diagnosis and management
Full guidance on nice.org.uk. BetterCall is not endorsed by NICE.
External resources
Educational reference only. Medical disclaimer