A patient taking oral doxycycline for scrub typhus asks whether he can take his antacid and iron supplement together with the antibiotic. The correct counseling is based on which property of tetracyclines?

• A They undergo extensive first-pass metabolism that polyvalent cations accelerate
• B They induce intestinal P-glycoprotein, increasing efflux of co-administered minerals
• C They are proton pump substrates whose uptake falls when gastric pH rises
• D They chelate divalent and trivalent cations, forming insoluble complexes that reduce absorption ✓

Explanation

Tetracyclines chelate divalent and trivalent cations such as calcium, magnesium, aluminum, and iron, forming insoluble nonabsorbable complexes, so dairy products, antacids, and iron preparations markedly reduce their bioavailability. Doxycycline and minocycline are less affected than older tetracyclines but separation of doses is still advised. The problem is chemical complexation in the gut lumen, not altered gastric pH or transporter induction.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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Written and medically reviewed by the StethoPrep medical team.