Drug Interaction Report

ADJUST DOSING INTERVAL: The bioavailability of oral tetracyclines may be significantly decreased during concurrent administration with zinc-containing products. Therapeutic failure may result. The proposed mechanism is chelation of tetracyclines by the zinc cation, forming an insoluble complex that is poorly absorbed from the gastrointestinal tract. In seven healthy volunteers, simultaneous oral administration of zinc sulfate 200 mg and tetracycline 500 mg resulted in a 30% to 40% reduction in tetracycline serum concentrations compared to administration of tetracycline alone. In the same study, doxycycline absorption was not significantly affected by zinc coadministration. Other studies have reported reductions of greater than 50% in tetracycline absorption in the presence of zinc salts.

MANAGEMENT: Patients receiving an oral tetracycline in combination with zinc-containing products should be advised to separate the times of administration by at least three to four hours. Doxycycline may be an appropriate alternative, since its absorption may not be significantly affected by zinc coadministration.