Which B12: methyl, adenosyl, or hydroxocobalamin?

How it works

All four forms interconvert intracellularly. Cyanocobalamin and hydroxocobalamin are reduced enzymatically and converted to methylcobalamin or adenosylcobalamin as needed. Methylcobalamin and adenosylcobalamin can be reduced back to free cobalamin. Cellular cobalamin metabolism is tightly regulated; supplemental form is largely irrelevant to which cofactor reaches its enzyme.

Effective dose

Doses of 5000 mcg/day commonly seen in OTC products are above the trial-studied range without additional evidence of clinical benefit at that dose. Methylcobalamin and adenosylcobalamin tablets are widely sold OTC in UK but not BNF-licensed; doses vary widely (typical OTC 500-5000 mcg/day). Combined methylcobalamin plus adenosylcobalamin products: no superiority evidence over single forms.

Forms compared

Methylcobalamin parenteral: not a routine UK option; not preferred over hydroxocobalamin for any clinical indication based on current evidence. Adenosylcobalamin parenteral: not used in routine UK practice. Sublingual products: same passive-diffusion gut mechanism as swallowed oral; no meaningful superiority over swallowed cyanocobalamin at the same dose.

Timing

Reassessment: clinical symptoms, FBC including reticulocyte count, MCV trajectory, MMA where available. Routine re-checking serum B12 once on supplementation is not informative (level rises mechanically with dose). Pregnancy: continue B12 replacement started pre-conception through pregnancy and breastfeeding.

Safety profile

Theoretical caveats: severe renal impairment reduces thiocyanate clearance; heavy smokers have higher background cyanide load; very-high-dose cyanocobalamin (above 5 mg/day chronically) has not been studied for cyanide-specific endpoints. Hydroxocobalamin therapeutic context: 5 g IV (Cyanokit) for cyanide poisoning is 5000 times the supplemental dose and irrelevant to ordinary B12 supplementation; the hydroxocobalamin-for-detoxification supplement marketing framing does not reflect a meaningful clinical effect at supplement doses. The folate trap is the dominant safety concern: do not supplement folate without B12 when B12 deficiency is suspected.

Special populations

Confirmed pernicious anaemia or autoimmune gastritis: hydroxocobalamin IM lifelong replacement. Post-gastrectomy or terminal-ileal-resection: hydroxocobalamin IM. Severe renal impairment: theoretical cyanocobalamin caveat at very high doses; hydroxocobalamin or methylcobalamin reasonable alternative. Heavy smokers: higher background cyanide load; theoretical caveat at very high cyanocobalamin doses. Vegans and strict vegetarians: form choice on cost and tolerability; cyanocobalamin oral is BNF-licensed and cost-effective.

Interactions

Nitrous oxide exposure (recreational or anaesthetic): inactivates B12 by oxidising the cobalt centre across all cobalamin forms; can precipitate functional B12 deficiency. Folate co-supplementation during acute B12 repletion: folic acid 5 mg/day for 4 weeks is sometimes added to address functional folate deficiency from accelerated red cell production, but B12 must be confirmed and repletion started first or alongside.

Guideline positions

Thakkar and Billa 2015: trials have not consistently demonstrated clinical superiority of methylcobalamin over cyanocobalamin or hydroxocobalamin for haematological response, neurological response, or symptom correction. Adenosylcobalamin specifically: essentially no trial evidence supporting clinical superiority over other forms for any specific indication. Combined methylcobalamin + adenosylcobalamin supplements: no trial evidence of superiority over single-form preparations. Cochrane evidence (Wang 2018) is on cyanocobalamin oral, not specifically on methyl or adenosyl forms.

Practical framework

No clinical indication specifically requires methylcobalamin or adenosylcobalamin over cyanocobalamin or hydroxocobalamin in routine UK practice. Default to UK BNF / NICE-licensed forms (hydroxocobalamin parenteral, cyanocobalamin oral) for clinical indications; OTC supplement form is largely a personal-preference question. This is a summary of published research, not personal health advice. Discuss any health or supplement decisions with a qualified healthcare professional, particularly during ongoing care, pregnancy, or with chronic conditions.

Common misconceptions

Claim: hydroxocobalamin is preferred for detoxification or cyanide-binding at supplement doses. The therapeutic Cyanokit context (5 g IV) is irrelevant to supplement doses. Hydroxocobalamin UK first-line parenteral status is based on retention duration enabling longer dosing intervals, not detoxification properties.

Claim: sublingual is superior to swallowed. Both rely on passive intestinal diffusion.

Claim: combined methyl plus adenosyl is comprehensive support. No superiority evidence over single forms.

Claim: methylcobalamin causes overstimulation in COMT-variant carriers (anxiety, insomnia). Functional-framing claim not anchored in trial data.