Calcium carbonate can interfere with the absorption of which class of antibiotics?

• A. Tetracyclines
• B. Fluoroquinolones
• C. Macrolides
• D. Beta-lactams

Answer: (A)

Calcium carbonate reduces absorption of tetracyclines because calcium ions bind to the antibiotic in the gut to form a calcium-tetracycline chelate. This chelated complex is poorly absorbed, so less drug enters the bloodstream and its effectiveness drops. The key idea is that this interaction happens in the intestinal lumen and is due to chelation with a divalent cation, not because the drug is broken down or degraded.

Macrolides and beta-lactams aren’t reliant on the same chelation-dependent absorption, so calcium around the time of those antibiotics doesn’t typically cause the same reduction in absorption. Fluoroquinolones can also be affected by divalent cations, but this question highlights tetracyclines as the classic example. To avoid the interaction, separate the dosing times by several hours.