Pills, Sprays, or Shots? The Best Way to Restore Vitamin B12 May Not Need a Needle

Vitamin B12 deficiency affects millions of people worldwide, from vegans and older adults to patients with diabetes taking metformin and those who have undergone bariatric or gastric surgery. For decades, the default treatment has been a needle: intramuscular injections that bypass the gut and deliver cobalamin directly into muscle tissue. But a new systematic review published in eClinicalMedicine suggests that the long-standing hierarchy of supplementation routes may be far less clear-cut than clinicians have assumed. After synthesizing 13 comparative studies spanning more than two decades and four continents, researchers found that oral, sublingual, and intramuscular vitamin B12 all reliably raise serum levels of the vitamin, and that no route has ever demonstrated superiority for the outcomes that matter most to patients.

The review, led by Daniela Debone and colleagues and prospectively registered with PROSPERO, searched PubMed, Embase, and Cochrane CENTRAL from database inception to July 2025, with an update in July 2026, imposing no language or date restrictions. The team screened 4,543 records, removed 1,499 duplicates, and ultimately included 13 studies: seven randomized controlled trials, three observational cohorts, two conference abstracts, and one research letter. The studies came from Japan, Israel, Turkey, the United States, Switzerland, Brazil, Spain, India, Austria, and New Zealand, with sample sizes ranging from just 16 participants to 4,281. Because the studies varied so widely in design, dosing, populations, and outcome reporting, the authors performed a descriptive synthesis rather than a statistical meta-analysis.

The physiological stakes are considerable. Vitamin B12, or cobalamin, is indispensable for DNA synthesis, methylation reactions, and the formation of myelin, the insulating sheath around nerve fibers. It also serves as a cofactor in mitochondrial energy metabolism and in the maturation of blood cells. Because the human body cannot synthesize it, cobalamin must come from animal-derived foods or fortified products. When deficiency sets in, the consequences can include megaloblastic anemia, peripheral neuropathy, cognitive impairment, and elevated cardiovascular risk. Populations at particular risk include children, vegetarians and vegans, people with pernicious anemia, patients on long-term metformin, and those who have lost part or all of their stomach.

Intramuscular injection has traditionally been favored because it bypasses the gastrointestinal tract entirely, guaranteeing systemic delivery even when gut absorption is impaired. Yet injections carry real costs: pain, the need for a healthcare professional, and repeated clinic visits. Oral supplementation has long faced skepticism, particularly in pernicious anemia, where the absence of intrinsic factor cripples active absorption. The counterargument rests on passive diffusion: roughly 1% of orally ingested cobalamin can be absorbed independent of intrinsic factor, which is why high-dose oral regimens of 1 to 2 milligrams per day can work even in malabsorptive conditions. Sublingual formulations, meanwhile, are absorbed through the highly vascularized oral mucosa, entering systemic circulation directly and avoiding hepatic first-pass metabolism.

The biochemical evidence assembled in the review is strikingly consistent in one direction: all three routes work. In the landmark trial by Kuzminski and colleagues, patients receiving 2 milligrams of oral cyanocobalamin daily actually achieved higher serum levels after four months (1,005 pg/mL) than those receiving monthly 1-milligram injections (325 pg/mL), a statistically significant difference. In contrast, the Swiss trial by Metaxas and colleagues found the opposite pattern over 28 days, with 100% of intramuscular patients normalizing their levels compared with 84.2% of oral patients, and mean concentrations of 2,796 pmol/L versus 354 pmol/L. The Spanish OB12 non-inferiority trial in older adults found essentially equivalent normalization rates at eight weeks, with a slight per-protocol edge for injections at 52 weeks that did not hold up under intention-to-treat analysis.

The sublingual route, though underrepresented with only five studies, produced some of the review’s most intriguing findings. In a large Israeli retrospective cohort of 4,281 patients, sublingual tablets produced significantly larger mean increases in serum B12 than injections (252 versus 218 ng/L) and higher odds of achieving increased levels. In a New Zealand trial of metformin-treated patients with type 2 diabetes, sublingual methylcobalamin outperformed a single hydroxocobalamin injection at three months, though the advantage vanished by six months. Head-to-head comparisons of sublingual versus oral supplementation, in Israel and India, found no meaningful differences: in the Indian trial, normalization rates were 68.4% for sublingual and 66.7% for oral after three months of 1,500 micrograms daily, with compliance above 90% in both arms.

Yet the review’s authors are careful to sound a note of caution that cuts through the numbers: higher serum B12 does not automatically mean better clinical outcomes. The magnitude of a biochemical response does not necessarily translate into greater symptom relief, neurological recovery, or quality-of-life improvement. Indeed, no study in the review demonstrated superiority of any route for long-term clinical outcomes, largely because clinical endpoints were rarely measured at all. The certainty of the evidence was further constrained by open-label designs, unadjusted confounding in the observational studies, which were judged to be at serious risk of bias, and inconsistent outcome reporting, with some trials lacking formal statistical comparisons altogether.

What the evidence does support is a shift in how route selection might be framed. When biochemical responses are broadly similar, the decision can reasonably be guided by patient preference, tolerability, and access rather than presumed efficacy differences. Patient preference data from the review are telling: in the Spanish trial, 83.4% of participants preferred oral administration, including more than two-thirds of those actually receiving injections. In the Swiss trial, 45.9% initially favored oral treatment versus 21.6% for injections, though preferences shifted after experiencing treatment. Non-invasive options also reduce reliance on healthcare infrastructure, a meaningful advantage for adherence and for resource-limited settings where repeated clinic visits impose real burdens on patients and systems alike.

The review also refines, rather than overturns, a 2024 network meta-analysis that had ranked intramuscular administration highest for biochemical efficacy while finding no statistically or clinically significant differences between routes. The new synthesis improves on that work in several ways: it was prospectively registered, searched four years more recently, applied no language restriction, focused exclusively on adults rather than pooling pediatric and adult populations, and incorporated conference abstracts and a research letter that the earlier review excluded. The authors note the review was funded by a pharmaceutical manufacturer and was undertaken to establish a shared evidence base for a subsequent Delphi consensus on B12 replacement strategies.

The gaps that remain are as instructive as the findings. Safety data were sparse across all routes, and none of the included studies reported adverse events for sublingual supplementation. Follow-up periods were mostly short to medium term, ranging from four weeks to 24 months. Future research, the authors argue, should include adequately powered, long-term randomized trials using standardized, patient-centered outcomes such as symptom improvement, adherence, and preference, particularly for the sublingual route. Until such trials arrive, the practical message for patients and clinicians is quietly radical: the shot in the arm that has defined B12 replacement for generations may be no better than a tablet dissolved under the tongue, and the choice may ultimately belong to the patient rather than the syringe.

Subject of Research: Comparative efficacy of oral, sublingual, and intramuscular vitamin B12 supplementation routes in adults

Article Title: Comparative efficacy of vitamin B12 supplementation routes in adults with replacement indication: a systematic review

Article References: Debone, D., Azeredo da Silva, A. F., Cordenonssi, J. T., & Cohen, R. V. (2026). Comparative efficacy of vitamin B12 supplementation routes in adults with replacement indication: a systematic review. eClinicalMedicine, 100, Article 104214. https://doi.org/10.1016/j.eclinm.2026.104214

Image Credits: AI Generated

DOI: 10.1016/j.eclinm.2026.104214

Keywords: vitamin B12, cobalamin, sublingual supplementation, intramuscular injection, oral supplementation, systematic review, vitamin B12 deficiency, pernicious anemia, metformin, patient preference, eClinicalMedicine, biochemical outcomes