NutriStack
Calcium and Famotidine, a caution.
Famotidine reduces gastric acid and can lower the absorption of calcium carbonate, which depends on acid to dissolve. The fracture-risk signal is weaker for H2 blockers than for PPIs, but the absorption effect is plausible and worth noting for patients who use calcium carbonate as their primary calcium source.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
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At a glance
- Substances
- Calcium and Famotidine
- Pair type
- Caution
- Evidence (highest tier)
- Moderate
- Source citations
- 2 sources
- Stack Score effect
- −5 to your Stack Score (per scored caution row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Caution · Moderate evidence
Caution
What is happening. Famotidine reduces gastric acid and can lower the absorption of calcium carbonate, which depends on acid to dissolve. The fracture-risk signal is weaker for H2 blockers than for PPIs, but the absorption effect is plausible and worth noting for patients who use calcium carbonate as their primary calcium source.
Mechanism. Calcium carbonate requires acid for dissolution and ionization to Ca2+ before duodenal absorption. Famotidine blunts gastric acid secretion via H2-receptor blockade, reducing dissolution efficiency.
Recommendation. If you take famotidine regularly, prefer calcium citrate, which absorbs well without gastric acid. If using calcium carbonate, take it with a meal when residual acid is highest.
Sources (2)
- Freedberg DE, Kim LS, Yang YX. The Risks and Benefits of Long-term Use of Proton Pump Inhibitors: Expert Review and Best Practice Advice From the American Gastroenterological Association. Gastroenterology. 2017;152(4):706-15. PMID 28257716
- Zhou B, Huang Y, Li H, Sun W, Liu J. Proton-pump inhibitors and risk of fractures: an update meta-analysis. Osteoporos Int. 2016;27(1):339-47. PMID 26462494
Stack Score
How this pair moves the number.
Effect on the composite score
If both Calcium and Famotidine are in the same stack, this pair applies −5 to your Stack Score (per scored caution row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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