Calcium supplements can interfere with absorption of which antibiotics?

• A. Macrolides and penicillins
• B. Tetracyclines and fluoroquinolones
• C. Cephalosporins
• D. Aminoglycosides

Answer: (B)

Calcium supplements can bind to certain antibiotics in the gut, forming insoluble complexes that keep the drug from being absorbed into the bloodstream. This reduces the drug’s gut bioavailability and can lower its effective concentration. The antibiotics most affected are tetracyclines and fluoroquinolones, because their chemical structures readily chelate with calcium. That means taking calcium at the same time as these drugs can make them less effective. To avoid this, separate dosing so the antibiotic is taken a couple of hours before calcium or several hours after (commonly about 2 hours before or 4 hours after, depending on the specific drug). Examples include doxycycline or other tetracyclines, and ciprofloxacin or levofloxacin. Other antibiotic classes like macrolides, penicillins, cephalosporins, and aminoglycosides don’t have the same significant interaction with calcium in terms of absorption.