Calcium (oral salts: carbonate, citrate, lactate, gluconate) × Iron oral (salts: sulfate, bisglycinate, fumarate, gluconate, polymaltose; heme iron polypeptide, lactoferrin)
ModerateMineral supplements. Calcium×Oral iron preparations; antianaemic agents
Updated · Methodology
Mechanism
Calcium interferes with the absorption of both heme and non-heme iron, and it is the only mineral described with that dual effect. They compete for enterocyte transporters, and calcium also acts inside the cell on iron's exit into the blood. Competition peaks on an empty stomach, which is exactly how iron is meant to be taken.
Symptoms
Ferritin rising slowly despite correct iron dosing, with fatigue that persists. In pregnancy and in women with heavy periods the consequence arrives sooner, because the reserve margin is smaller.
Management
Space them by at least two hours: iron in the morning on an empty stomach with vitamin C, calcium with lunch or dinner. Complexes combining iron and calcium in one capsule deliver less than the label indicates. During anaemia treatment, calcium does not go in the same dose as iron.
Check the full regimen, not just this pair
Opens the checker with these two drugs prefilled. Add the rest of the regimen and recompute additive risks.
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Sources
- NIH Office of Dietary Supplements: Iron – Health Professional Fact Sheet (2025)– NIH ODS, Iron, Interactions with Medications
- NIH Office of Dietary Supplements: Calcium – Health Professional Fact Sheet (2025)– NIH ODS, Calcium, Interactions with Medications