Vitamin B12 Injection

Vitamin B12 (cobalamin) is essential for DNA synthesis, red-blood-cell formation, and neurological function. Deficiency in adults is most often caused by malabsorption — atrophic gastritis with age, gastric or ileal surgery, autoimmune pernicious anemia, or chronic PPI / metformin use — rather than dietary inadequacy alone.¹ Strict plant-based diets without supplementation are the main dietary cause.²

Why injection over oral

For most patients, high-dose oral B12 (1–2 mg daily) is comparable to injections for correcting uncomplicated deficiency.³ Injection has three remaining advantages: faster repletion when deficiency is symptomatic, certain absorption in patients with severe malabsorption or pernicious anemia who lack functional intrinsic factor, and patient compliance when daily oral dosing is hard to maintain.⁴

Who benefits

• Documented serum B12 deficiency, especially with elevated methylmalonic acid (MMA) confirming functional deficiency.
• Pernicious anemia (autoimmune destruction of intrinsic factor).
• Post-bariatric surgery patients, especially after gastric bypass or sleeve gastrectomy.
• Strict plant-based diet without B12 supplementation.
• Long-term metformin or PPI use with elevated MMA.

What to expect

When deficiency is real, energy and cognitive symptoms often improve within 1–2 weeks. Lab markers normalize within 4–8 weeks. In men without documented deficiency, the literature is consistent: B12 supplementation does not produce measurable benefit.³ If labs are normal and symptoms are vague, the right next step is workup for other causes — not empirical B12 injections.