Which antacid ingredient can interfere with absorption of tetracycline antibiotics?

• A. Calcium carbonate
• B. Magnesium hydroxide
• C. Simethicone
• D. Aluminum hydroxide

Answer: (A)

Tetracycline antibiotics can form chelate complexes with divalent and trivalent metal ions in the gut, which prevents the drug from being absorbed. Calcium ions, in particular, readily bind tetracycline and create an insoluble complex that the body cannot efficiently take up. Calcium from calcium carbonate-containing antacids provides those calcium ions, so the interaction is strong and reduces tetracycline bioavailability.

Simethicone doesn’t affect absorption; it’s just an anti-foaming agent. Magnesium and aluminum-containing antacids can also chelate tetracycline, but calcium-containing antacids are a classic and commonly tested example of this interaction. To minimize this, separate dosing of tetracycline and calcium-containing antacids by several hours.