If I am taking oral iron supplementation, should I also receive a vitamin B12 injection?

Should You Take a B12 Shot with Oral Iron Supplementation?

No, there is no evidence-based reason to combine vitamin B12 injections with oral iron supplementation unless you have a documented B12 deficiency or a specific condition causing B12 malabsorption. Iron and B12 are distinct nutrients with separate absorption pathways, and taking one does not require supplementation of the other. 1

When B12 Injections Are Actually Indicated

B12 injections are reserved for patients with confirmed malabsorption conditions, not for routine use alongside iron therapy. 2

Mandatory indications for B12 injections include:

• Pernicious anemia (positive anti-intrinsic factor antibodies) 2
• Post-bariatric surgery (Roux-en-Y gastric bypass, sleeve gastrectomy, biliopancreatic diversion) requiring 1000 µg IM monthly for life 1, 2
• Ileal resection >20 cm requiring prophylactic 1000 µg IM monthly indefinitely 2
• Crohn's disease with >30-60 cm ileal involvement 2
• Neurological symptoms from B12 deficiency (paresthesias, gait disturbance, cognitive impairment) requiring hydroxocobalamin 1 mg IM on alternate days until improvement plateaus 2

When oral B12 is sufficient:

• Dietary insufficiency (vegans, vegetarians) responds well to oral supplementation of 1000-2000 µg daily 3, 4
• Metformin use >4 months can be managed with oral B12 3
• PPI or H2-blocker use >12 months typically responds to oral therapy 3

Iron and B12: Separate Absorption Pathways

Iron absorption occurs primarily in the duodenum and proximal jejunum, while B12 absorption requires intrinsic factor binding in the stomach and absorption in the terminal ileum. 1 These are completely independent processes:

• Iron absorption is enhanced by vitamin C, not B12, so taking iron with citrus fruits or vitamin C supplements improves uptake 1
• Iron and calcium should be separated by 1-2 hours to avoid interference, but B12 has no such interaction 1
• Taking B12 with iron provides no absorption benefit for either nutrient 1

When Both Deficiencies Coexist

Iron deficiency and B12 deficiency can occur together in specific conditions, but this requires treating each deficiency independently based on laboratory confirmation. 5

High-risk populations for concurrent deficiencies:

• Post-bariatric surgery patients have permanent malabsorption affecting both nutrients and require lifelong supplementation of each 1, 2
• Pernicious anemia patients have 75% prevalence of iron deficiency due to atrophic gastritis raising gastric pH and impairing iron absorption 5
• Crohn's disease with ileal involvement affects B12 absorption and may cause iron deficiency from chronic inflammation or bleeding 2

Treatment approach when both are deficient:

• Treat B12 deficiency first if neurological symptoms are present, as giving folic acid or iron before correcting B12 can mask megaloblastic anemia while allowing irreversible subacute combined degeneration of the spinal cord to progress 1, 2
• After initiating B12 therapy, add iron supplementation with 200 mg ferrous sulfate, 210 mg ferrous fumarate, or 300 mg ferrous gluconate daily 1
• In pernicious anemia with iron deficiency, intravenous iron is more effective than oral (7/8 patients normalized iron status with IV versus 3/11 with oral supplementation at 3 months) 5

Critical Pitfalls to Avoid

Do not assume you need B12 injections simply because you are taking iron. 1 The most common mistakes include:

• Starting B12 without documented deficiency (serum B12 <180 pg/mL or <133 pmol/L confirms deficiency) 2, 6
• Using injections when oral B12 would suffice – oral cyanocobalamin 1000-2000 µg daily is as effective as IM for most patients without severe neurological symptoms 3, 4, 7
• Giving folic acid before treating B12 deficiency, which can precipitate irreversible neurological damage 1, 2
• Assuming "normal" B12 levels rule out deficiency – up to 50% of patients with normal serum B12 have metabolic deficiency when methylmalonic acid is measured 6

Practical Algorithm for Decision-Making

Follow this stepwise approach:

1. If you are taking iron for documented iron deficiency, continue iron alone unless you have symptoms suggesting B12 deficiency (fatigue, paresthesias, cognitive difficulties, glossitis) 2, 6

2. If you have risk factors for B12 deficiency (age >75 years, metformin >4 months, PPI >12 months, bariatric surgery, vegan diet, ileal disease), get serum B12 tested 3

3. If serum B12 <180 pg/mL, you need treatment – but oral B12 1000-2000 µg daily is effective for most patients 3, 4, 7

4. Injections are required only if you have confirmed malabsorption (pernicious anemia, post-bariatric surgery, ileal resection >20 cm) or severe neurological symptoms 2

5. Monitor both nutrients separately – check iron studies (ferritin, transferrin saturation) and B12 levels at 3,6, and 12 months, then annually 2

The bottom line: Taking a B12 shot with iron supplementation is unnecessary unless you have a documented B12 deficiency from malabsorption or a high-risk condition requiring lifelong B12 therapy. 2, 3