Supplements are not tested in clinical trials and are not registered as medications.
Mechanism of action
Cyanocobalamin (vitamin B12) is a cofactor for two enzymes: methionine synthase (remethylation of homocysteine to methionine) and methylmalonyl-CoA mutase (branched-chain fatty acid metabolism). It is required for DNA synthesis, myelination of nerve fibres, and erythrocyte maturation. Deficiency impairs erythrocyte precursor division (megaloblastic haematopoiesis) and causes demyelination of the posterior and lateral spinal cord columns.
Indications
A
B12 deficiency with neurological symptoms
First line
In subacute combined degeneration and other neurological features of B12 deficiency, high-dose parenteral cyanocobalamin or hydroxocobalamin is the only disease-modifying therapy. Maximal recovery occurs in the first 6 months, before irreversible axonal degeneration develops. With late treatment, neurological deficit may partially persist. Early suspicion of B12 deficiency prompts B12 and homocysteine testing without delaying therapy until results return.
Cyanocobalamin or hydroxocobalamin is first-line for B12 deficiency anaemia. Standard regimen for pernicious anaemia: 1,000 µg intramuscularly daily for 1 week, weekly for 4 weeks, then monthly for life. Oral therapy uses 1,000–2,000 µg daily; the 2018 Cochrane review showed comparable efficacy of oral and parenteral routes at high oral doses. International haematology societies ( 2014) recognise both routes.
In pernicious anaemia with neurological features, parenteral administration is preferred in the first 2–4 weeks for rapid repletion.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
F
Chronic fatigue
Not recommended
Cyanocobalamin (vitamin B12) is a water-soluble vitamin. It is prescribed for confirmed B12 deficiency (megaloblastic anemia, after ileal resection, pernicious anemia). For chronic fatigue without confirmed B12 deficiency, cyanocobalamin does not work: randomized clinical trials in people with normal serum B12 have shown no benefit. The marketing of B12 injections for energy in anti-aging clinics has no evidence base. B12 deficiency should be confirmed by blood test (B12 below 200 pg/mL or borderline with elevated homocysteine/MMA). If cyanocobalamin was prescribed for fatigue without a blood test, consider seeking a second opinion and checking your B12 level.
Oral forms are taken regardless of meals. Sublingual forms are placed under the tongue until dissolved – additional benefit over swallowing in people with low gastric acidity or atrophic gastritis is not proven, as intestinal absorption at high doses is comparable.
Dose titration
Deficiency treatment: 1,000–2,000 µg daily orally or 1,000 µg intramuscularly on schedule (pernicious anaemia). Prophylaxis in vegans and metformin users: 500–1,000 µg daily 1–2 times weekly or 250 µg daily. Physiological adult daily requirement is 2.4 µg – hundreds of times less than therapeutic doses because oral absorption is low (1–2% without intrinsic factor).
Monitoring
4–8 weeks after starting therapy, check complete blood count (haemoglobin, MCV) and serum B12. Haemoglobin normalises over 4–8 weeks, MCV over up to 3 months. Neurological symptoms improve more slowly – over 6–12 months. Homocysteine and methylmalonic acid are more sensitive deficiency markers, used in unclear cases.
Special situations
Patients on metformin for more than 4 years, with atrophic gastritis, after gastrectomy or ileal resection, with chronic pancreatitis, or with small intestinal bacterial overgrowth are at high risk. Vegans without supplementation inevitably develop deficiency within 3–5 years. In adults over 65, deficiency prevalence reaches 20%.
Common myths
Drug–nutrient interactions
Folate
Concurrent B12 and folate therapy is required in megaloblastic anaemia of unclear aetiology. Isolated high-dose folic acid in unrecognised B12 deficiency masks macrocytosis but does not stop neurological degeneration. B12 deficiency must be excluded before prescribing folate.
Drugs for peptic ulcer and GORD. Proton pump inhibitors
Mechanism
Esomeprazole suppresses gastric acidity, reducing release of B12 from food (which requires HCl and pepsin). With over 2 years of therapy, B12 deficiency risk is reported.
Symptoms
Fatigue, pallor, paraesthesias in hands and feet, gait instability, memory decline. Develops over 2–4 years of unrecognised therapy.
Management
On long-term PPIs (over 2 years), check B12 yearly. Oral cyanocobalamin tablets or drops can be given regardless of acidity (passive diffusion absorption at high doses). Alternative: H2-blockers (famotidine) for short-term acid suppression.
Hypersensitivity to cyanocobalamin or other components
Polycythaemia vera, erythrocytosis
Thromboembolic disease (relative)
Serious adverse effects
Anaphylactic reactions (very rare, more common with repeated injections)
Common adverse effects
Injection site pain
Allergic reactions (urticaria, pruritus)
Uncommon adverse effects
Headache, dizziness
Dyspepsia
Acneiform skin eruptions with high-dose parenteral use
PregnancyFDA A
Safe at recommended doses. Pregnant vegans and women with confirmed deficiency require supplementation – maternal B12 deficiency increases the risk of neural tube defects and developmental delay in the foetus.
Breastfeeding
Transfers into breast milk. Maternal B12 deficiency (especially in vegan mothers) causes infant deficiency during breastfeeding – maternal supplementation is required.
Cyanocobalamin is evaluated for the following indications with varying evidence strength: Vitamin B12 deficiency anaemia (pernicious anaemia) (evidence tier A), B12 deficiency with neurological symptoms (evidence tier A), Chronic fatigue (evidence tier F). See the full indication matrix with dosing and citations above on this page.
What are the side effects of Cyanocobalamin?
Common side effects of Cyanocobalamin (≥ 1 in 100): Injection site pain, Allergic reactions (urticaria, pruritus). See the Safety section for uncommon and serious reactions.
Is Cyanocobalamin safe during pregnancy?
FDA category A. Safe at recommended doses. Pregnant vegans and women with confirmed deficiency require supplementation – maternal B12 deficiency increases the risk of neural tube defects and developmental delay in the foetus.
Is Cyanocobalamin compatible with breastfeeding?
Transfers into breast milk. Maternal B12 deficiency (especially in vegan mothers) causes infant deficiency during breastfeeding – maternal supplementation is required.
Who should not take Cyanocobalamin?
Cyanocobalamin is contraindicated in: Hypersensitivity to cyanocobalamin or other components; Polycythaemia vera, erythrocytosis; Thromboembolic disease (relative). Full list in the Safety section.
Myth: “B12 injections give energy”. Fact: in patients with normal B12, there is no effect on energy or performance. Popular vitamin cocktails and B12 drips for healthy people lack evidence and carry injection-related risks.
Myth: “methylcobalamin is better than cyanocobalamin”. Fact: both forms are effective for deficiency correction. Methylcobalamin is more expensive, and direct RCTs showing superiority are absent. Cyanocobalamin converts to active coenzyme forms in the body.
Myth: “oral treatment doesn't work in pernicious anaemia”. Fact: the 2018 Cochrane review showed comparable efficacy of high-dose oral and parenteral routes. In neurological involvement and severe anaemia, injections are preferred in the first 2–4 weeks, then switching to tablets is possible.
Metformin suppresses calcium-dependent absorption of the B12-intrinsic factor complex in the terminal ileum. With over 4 years of therapy, clinically significant B12 deficiency is reported in 10–30% of patients.
Symptoms
Fatigue, pallor, paraesthesias in hands and feet, gait instability, memory decline. Develops over 2–4 years of unrecognised therapy.
Management
On long-term metformin, check B12 yearly. If it falls below 200 pg/mL, give cyanocobalamin 500–1000 mcg/day orally or 1000 mcg/week intramuscularly (in significant neuropathy). Supplementation does not cancel the metformin indication.
Pantoprazole reduces gastric acidity and B12 release from food. With long-term therapy, B12 deficiency risk is reported, similar to other PPIs.
Symptoms
Fatigue, pallor, paraesthesias in hands and feet, gait instability, memory decline. Develops over 2–4 years of unrecognised therapy.
Management
On long-term pantoprazole (over 2 years), check B12 yearly. Cyanocobalamin can be given regardless of acidity (passive diffusion absorption at high doses). For sustained acid suppression, consider dose reduction or periodic breaks.
Proton pump inhibitors suppress gastric acidity and reduce B12 release from food (which requires HCl and pepsin). For short courses (up to 8 weeks), clinically insignificant. With long-term therapy (over 2 years), B12 deficiency risk is reported.
Symptoms
On long-term therapy: fatigue, pallor, paraesthesias in hands and feet, gait instability, memory decline.
Management
For short omeprazole courses, no adjustment needed. On long-term therapy (over 2 years), check B12 yearly. If it falls below 200 pg/mL, give cyanocobalamin 500–1000 mcg/day orally — passive diffusion absorption at high doses works regardless of acidity.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
“B12 injections give energy”
in patients with normal B12, there is no effect on energy or performance. Popular vitamin cocktails and B12 drips for healthy people lack evidence and carry injection-related risks.
“methylcobalamin is better than cyanocobalamin”
both forms are effective for deficiency correction. Methylcobalamin is more expensive, and direct RCTs showing superiority are absent. Cyanocobalamin converts to active coenzyme forms in the body.
“oral treatment doesn't work in pernicious anaemia”
the 2018 Cochrane review showed comparable efficacy of high-dose oral and parenteral routes. In neurological involvement and severe anaemia, injections are preferred in the first 2–4 weeks, then switching to tablets is possible.