Background: The growing frequency of vitamin B12 insufficiency in people with diabetes mellitus raises major question for haematological health. This study sought to assess the peripheral blood picture—specifically, the presence of cytopenias and neurological symptoms—across the spectrum of vitamin B12 insufficiency severity in a group of type 2 diabetic patients.From January 2024 to September 2024, a cross-sectional, hospital-based study was carried out in a tertiary care centre in Jharkhand, India. The sample size determined was 420. Informed consent was given by adult type 2 diabetes mellitus diagnosed patients with serum vitamin B12 levels < 180 pg/mL. Demographic, clinical history, metformin use, vegetarian diet, vitamin B12 levels, full blood counts, and neurological symptoms—tingling, numbness, ataxia—were all gathered. Deficiency (100-179 pg/mL) and Severely Deficient (<100 pg/mL) were classifications for vitamin B12 deficiency. The adjusted relationship between severe vitamin B12 insufficiency and the results of anaemia, macrocytosis, leukopenia, thrombocytopenia, and presence of any neurological symptom was evaluated using multivariate logistic regression.There were 420 type 2 diabetic patients lacking vitamin B12 in the study. After controlling for age, sex, vegetarian diet, and metformin use, severe vitamin B12 insufficiency was notably linked to a higher likelihood of macrocytosis (Adjusted OR: 4.18, 95% CI: 2.03-8.61, p < 0.001). Although trends were seen, adjusted analysis revealed no significant correlation between severe B12 deficiency and anaemia, leukopenia, thrombocytopenia, or any reported neurological symptom.Conclusion: Among type 2 diabetic patients lacking vitamin B12 in this cohort, severe deficiency was a strong independent predictor of macrocytosis. The link between vitamin B12 insufficiency severity and other haematological parameters and neurological symptoms in this vulnerable population has to be clarified by more study using larger sample sizes and longitudinal studies.