Background <p>Vitamin B12 deficiency is a common health concern, often linked to Helicobacter pylori (H. pylori) infection. This study aimed to quantify the prevalence of vitamin B12 deficiency among H. pylori-positive patients with functional dyspepsia and to evaluate changes in vitamin B12 levels before and after eradication therapy.</p> Methods <p>A total of 179 patients with functional dyspepsia were enrolled in this hospital-based observational study. Demographic, clinical, and socioeconomic data were collected. Serum vitamin B12 levels were measured before and after H. pylori eradication therapy. Associations between deficiency and patient characteristics were analyzed.</p> Results <p>Vitamin B12 deficiency was observed in 21.2% of patients and was significantly associated with older age (41–60 years) and hypertension. Lower income and limited education were also linked to higher deficiency rates. Post-eradication, mean vitamin B12 levels increased significantly from 126.72 ± 12.48 pg/mL to 328.41 ± 10.62 pg/mL (<i>p</i> = 0.01).</p> Conclusion <p>H. pylori infection contributes to moderate vitamin B12 deficiency, particularly among socioeconomically disadvantaged and older urban populations. Eradication therapy significantly improves B12 levels. Limitations include the absence of dietary data and functional B12 markers, which should be addressed in future studies.</p>