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

Background: Vitamin B12 deficiency is common and often requires long-term supplementation to prevent serious complications, yet the comparative efficacy of the oral, sublingual, and intramuscular routes remains uncertain. This systematic review evaluated the efficacy of different vitamin B12 supplementation routes in adults with an indication for replacement. Methods: PubMed, Embase, and Cochrane CENTRAL were searched without language or date restrictions, from inception to July 18, 2025, updated July 13, 2026, following PRISMA 2020 reporting. Eligible studies were randomized trials and observational cohorts comparing oral, sublingual, or intramuscular vitamin B12 supplementation in adults with an indication for replacement; pediatric and non-comparative studies were excluded. Risk of bias was assessed with RoB 2.0 (trials) and ROBINS-I (cohorts). The primary outcome was serum vitamin B12 or clinical response. This systematic review was registered with PROSPERO, number CRD420251126832. Findings: 13 studies met the inclusion criteria: 7 randomized controlled trials, 3 observational cohorts, 2 conference abstracts, and 1 research letter Included studies were conducted across 10 countries spanning Asia, Europe, North and South America, and Oceania, in both single-center and multicenter settings. Comparisons of oral vs. intramuscular supplementation were most frequent (8 studies), several suggesting comparable or superior biochemical responses with oral administration. Sublingual supplementation was underrepresented (5 studies) but showed short-term biochemical efficacy similar or superior to the other routes. All three routes increased serum vitamin B12 levels. All seven randomized controlled trials were judged to have some concerns regarding risk of bias, mainly due to limitations in randomization procedures and selective outcome reporting; all three observational studies were judged to be at serious risk of bias, primarily due to inadequate confounding adjustment. The remaining three studies (2 conference abstracts, 1 research letter) were not assessed. Interpretation: Route selection should consider patient preference, tolerability, and access. Evidence for sublingual supplementation remains particularly limited, although the available studies suggest that it may represent a potential non-invasive alternative for selected patients. Future research should include adequately powered comparative studies assessing patient-centered outcomes, such as symptom improvement, treatment adherence, and preference. Funding: Myralis Industria Farmacêutica Ltda.