Why Oral Iron and Antacids Should Not Be Taken Together
Antacids reduce oral iron absorption by raising gastric pH, which prevents the conversion of ferric iron to the more absorbable ferrous form and can cause precipitation of iron salts in the gastrointestinal tract. 1
Mechanism of Interaction
Iron absorption requires an acidic environment in the stomach for optimal uptake. Specifically:
- Gastric acid is essential for converting dietary non-heme iron (ferric form, Fe³⁺) into the more soluble and absorbable ferrous form (Fe²⁺) 2
- Antacids neutralize stomach acid, creating a higher pH environment that prevents this conversion and causes iron to precipitate as insoluble compounds 3, 4
- Aluminum-based phosphate binders (a type of antacid) specifically reduce iron absorption 1
Clinical Evidence of Reduced Absorption
The magnitude of this interaction varies by antacid type:
- Sodium bicarbonate reduces plasma iron increase by 50% compared to iron taken alone 3
- Calcium carbonate reduces plasma iron increase by 67% when taken simultaneously with iron 3
- Liquid antacids containing aluminum hydroxide and magnesium hydroxide cause marked reduction in iron absorption from all ferrous salt preparations 4
- Ferrous carbonate is particularly affected, becoming almost completely insoluble at neutral pH levels, with virtually no absorption in patients with achlorhydria 4
Proton Pump Inhibitors and H2 Blockers
The interaction extends beyond traditional antacids:
- Omeprazole and other proton pump inhibitors significantly impair oral iron absorption, with only 16% of patients achieving normal hemoglobin response (rise >2 g/dL) and only 40% achieving normal ferritin response (rise >20 μg/dL) after 3 months of ferrous sulfate therapy 5
- Combinations of PPIs, H2 blockers, and calcium carbonate have been clearly associated with development of iron deficiency anemia in prospective long-term follow-up studies 2
Important Exception
Calcium carbonate's inhibitory effect can be overcome when iron is taken in a multivitamin tablet containing ascorbic acid (vitamin C), as the competitive binding of iron by ascorbic acid allows uninhibited absorption 3
Clinical Recommendations
To optimize iron therapy:
- Separate administration times: Take oral iron at least 2 hours before or 1 hour after antacid use 1
- Take iron on an empty stomach when possible for maximum absorption 1
- Consider vitamin C supplementation (80-500 mg) with iron to enhance absorption and potentially overcome some antacid effects 1, 6
- Monitor response closely: If patients require both iron and acid suppression therapy, expect suboptimal response and consider higher doses, longer duration, or intravenous iron therapy 5
Common Pitfall
Do not assume all antacids equally affect iron absorption—while liquid aluminum/magnesium antacids show marked effects, the clinical significance may vary. However, the safest approach is to separate all antacid and iron administration times 3, 4