Timing of Vitamin B12 Supplementation After Iron
There is no required waiting period between taking iron and vitamin B12 supplements—they can be taken together without any clinically significant interaction affecting absorption or efficacy of either nutrient. 1
Key Evidence Supporting Concurrent Administration
The available clinical guidelines and research do not identify any pharmacokinetic or pharmacodynamic interaction between iron and vitamin B12 supplements that would necessitate separation of doses. 1
Iron Absorption Considerations
- Iron absorption is optimized when taken on an empty stomach in the morning with vitamin C (approximately 100 mg), not by separating it from B12. 1, 2
- The primary substances that impair iron absorption include calcium supplements, antacids, tea, coffee, milk, and food (especially grains and cereals)—vitamin B12 is not among these inhibitors. 1, 2
- Iron should be separated from calcium by at least 1-2 hours, as calcium directly inhibits iron absorption. 3
B12 Absorption Considerations
- Vitamin B12 absorption occurs primarily in the terminal ileum via intrinsic factor-mediated mechanisms and passive diffusion at high doses. 1, 4
- No evidence suggests that iron interferes with B12 absorption through either the intrinsic factor pathway or passive diffusion mechanisms. 1, 4
Clinical Context: Combined Deficiencies
Patients with iron deficiency anemia may have concurrent B12 deficiency, particularly in certain populations, and both should be assessed and treated simultaneously. 1
Important Diagnostic Sequence
- Always check and treat B12 deficiency BEFORE initiating folic acid supplementation, as folic acid can mask B12 deficiency anemia while allowing irreversible neurological damage to progress. 1, 5
- This critical warning applies to folate, not to iron—iron and B12 can be started together safely. 1
Populations at Risk for Combined Deficiencies
- Post-bariatric surgery patients (reduced gastric acid and intrinsic factor) 1, 3
- Patients with inflammatory bowel disease, especially with ileal involvement 1
- Chronic kidney disease patients 1
- Patients with pernicious anemia (75% have concurrent iron deficiency due to achlorhydria impairing iron absorption) 6
Practical Dosing Algorithm
For patients requiring both iron and B12 supplementation:
Morning dose (fasting): Take oral iron (ferrous sulfate 65 mg elemental iron) with 100 mg vitamin C or citrus juice, along with B12 supplement (1000-2000 mcg oral daily). 1, 4, 2
Avoid within 1-2 hours of iron: Calcium supplements, antacids, PPIs, H2 blockers, tea, coffee, milk, or food. 1, 3, 2
Remain upright for 30-60 minutes after taking iron to reduce risk of pill esophagitis. 2
Alternative timing if morning fasting is not tolerated: Iron can be taken with food for better tolerance, though absorption will be reduced; B12 can still be taken simultaneously as food does not significantly impair high-dose oral B12 absorption. 1, 4
Special Considerations
When IV Iron May Be Preferred
In pernicious anemia patients with concurrent iron deficiency, intravenous iron supplementation is more effective than oral iron (7/8 patients normalized iron status with IV vs. 3/11 with oral supplementation at 3 months), likely due to achlorhydria impairing oral iron absorption. 6