B12 Injections vs Sublingual Sprays

Why this question keeps coming up

Sublingual B12 sprays and lozenges have boomed in the UK supplement market. They're sold over the counter at Boots, Holland & Barrett, every major supermarket, and a hundred Amazon brands. They're cheap (a month's supply is typically £6–12), convenient, and don't involve needles. The marketing is heavy on 'as effective as injections' claims.

The clinical reality is more nuanced. Sublingual B12 absorption is real but modest. For some patients it's plenty. For others it's nowhere near enough. The answer depends on what's causing the deficiency.

How each route gets B12 into your blood

Intramuscular hydroxocobalamin (the injection)

100% bioavailability. The B12 goes straight into the deltoid muscle, into the circulation, and binds to transcobalamin for tissue delivery. No reliance on gut absorption. Hydroxocobalamin (the UK-licensed form) binds more tightly to plasma proteins than cyanocobalamin, which is why it stays in circulation longer and is dosed every 2–3 months instead of monthly.

Oral / sublingual B12 — active absorption pathway

Normal B12 absorption requires intrinsic factor (IF), a protein secreted by parietal cells in the stomach. IF binds dietary B12 in the duodenum and the complex is then absorbed at the terminal ileum. This is the 'active' pathway and accounts for ~99% of dietary B12 absorption in healthy people. If you have intact parietal cells, intact intrinsic factor, intact ileum, and no autoimmune disease blocking the process, oral B12 works.

Oral / sublingual B12 — passive absorption pathway

About 1% of any oral B12 dose is absorbed passively across the mucosa, independent of intrinsic factor. At physiological intakes (a few µg per day) this is negligible. At high pharmacological doses (1000–2000µg sublingual or oral), 1% becomes clinically meaningful — 10–20µg absorbed per dose, which is enough to maintain stable plasma B12 in patients without significant malabsorption.

Where the routes diverge

Pernicious anaemia

An autoimmune condition where intrinsic factor is destroyed or blocked by autoantibodies. The active absorption pathway is broken. Only the passive 1% remains, and even at high oral doses, the resulting absorption is unreliable. Crucially, the failure mode is silent — you may feel fine on sublingual sprays for months while your tissues continue to deplete, until neurological symptoms emerge that may not fully reverse. The British Society for Haematology guidelines and NICE CKS both recommend IM injection as the standard of care for pernicious anaemia.

Dietary deficiency

Strict vegans and many vegetarians can develop B12 deficiency simply because B12 is not produced by plants. The gut machinery is intact, so high-dose oral or sublingual B12 absorbs adequately. For these patients, sublingual sprays at 1000µg daily often maintain levels effectively. We often initiate with a short IM loading course (6 injections over 2 weeks) to rapidly correct deficiency, then transition to oral / sublingual maintenance.

Metformin and PPI-induced depletion

Metformin reduces B12 absorption by altering calcium-dependent ileal uptake. Long-term proton pump inhibitor (PPI) use reduces B12 release from dietary protein because gastric acid is needed to free B12 from food. Both mechanisms can be partially overcome with high-dose oral B12, but for patients with neurological symptoms or very low serum levels, IM is more reliable.

How we decide at Trafford Clinic

A 15-minute B12 consultation covers symptoms (fatigue, tingling, brain fog, balance), dietary history, current medications (metformin, PPIs, omeprazole, lansoprazole), and prior B12 results. We then draw serum B12, holotranscobalamin if indicated, folate, ferritin, and — if dietary explanation is absent — intrinsic factor antibodies. Results in 24–48 hours.

From there:

• Pernicious anaemia confirmed → IM hydroxocobalamin loading + 2–3 monthly maintenance, lifelong.
• Dietary deficiency, no neuro symptoms → IM loading (6 injections, 2 weeks) then transition to 1000µg daily sublingual / oral.
• Metformin/PPI-induced, no neuro symptoms → trial of high-dose oral first, retest at 3 months, escalate to IM if needed.
• Neurological symptoms regardless of cause → IM, no sublingual trial.

What's actually in your spray bottle?

Most UK sublingual sprays use cyanocobalamin (the cheaper synthetic form) rather than hydroxocobalamin. Both are converted to active methylcobalamin and adenosylcobalamin in the body, but cyanocobalamin requires removal of the cyanide group first. For most patients this is fine. Patients with smoker's optic neuropathy, severe renal impairment, or Leber's hereditary optic neuropathy should specifically use hydroxocobalamin or methylcobalamin.